Start/End GCODE: Help me understand why no worky

[Platyprintipus]

Hello,

First of all, thank you for your time.
Second, please consider me as someone with, at best, the most basic knowledge in 3D printing. I understand a few things, and picked up some basic techniques and tricks along the way.
Third, I'm printing with an Alfawise U20 Plus and use Cura.

So here's the dealio:
Regularly, I chance upon some designs that, when printed, fail at the very start. In spite of my efforts (preheating board and extruder, preheating more than required, using gluer/hair spray, ...) the first strands do not adhere to my board. At some point I thought it was a matter of speed or strand thickness, but apparently not.
Yesterday evening, I started messing around: instead of taking the gcode straight out of the designer's hands and feeding it to my machine, I started swapping the END/START sections with some "END/START always working" type of gcodes. Mind you, none worked, same results: strands start coalescing and everyting goes to hell.
Then I said to myself: Why not try it with the END/START gcode given by the default Alfawise U20 non-plus profile given by Cura. Lo and behold, my printing worked amazingly well.

Here's the original END/START GCODE for one of the pieces:

----------------------------------------------------- Original GCODE -----------------------------------------------------
; -- START GCODE --
G21 ;set units to millimetres
G90 ;set to absolute positioning
M106 S0 ;set fan speed to zero (turned off)
G28 ;home all axis
G92 E0 ;zero the extruded length
G1 Z1 F1000 ;move up slightly
G1 Y60.0 Z0 E9.0 F1000.0;intro line
G1 Y100.0 E21.5 F1000.0 ;continue line
G92 E0 ;zero the extruded length again
; -- end of START GCODE --
[Printy mumbojumbo]
; -- END GCODE --
M104 S0 ;turn off nozzle heater
M140 S0 ;turn off bed heater
G91 ;set to relative positioning
G1 E-10 F300 ;retract the filament slightly
G90 ;set to absolute positioning
G28 X0 ;move to the X-axis origin (Home)
G0 Y280 F600 ;bring the bed to the front for easy print removal
M84 ;turn off stepper motors
; -- end of END GCODE --

----------------------------------------------------- Superduper always (not) working internet GCODE -----------------------------------------------------
; -- START GCODE --
G21 ; Valeurs métriques
G90 ; Mode position absolue
M190 S50 R60 ; Chauffer le lit pour un minimum ?* 50°c et un maximum ?* 60°c
M109 S200 R210 ; Chauffer la buse pour un minimum ?* 200°c et un maximum ?* 210°c
G1 Z10 F2400 ; Placer la buse ?* 10mm du lit ?* 40mm/s
G28 X0 Y0 ; Origine 0 en X et Y
G28 Z0 ; Origine 0 en Z
G1 Z100 F2400 ; Placer la buse de 100mm du lit ?* 40mm/s
M83 ; Mode position relative pour l'extrudeur
G1 F240 E20 ; Extruder 20mm de fil pour "purger"
M82 ; Mode position absolue pour l'extrudeur
G92 E0 ; Réinitialiser l'origine ?* 0 de l'extrudeur
G4 S10 ; Temps d'attente de 10 sec pour bien nettoyer ce qui est sorti et sort encore de la buse
G1 Z10 F2400 ; Placer la buse ?* 10mm du lit ?* 40mm/s
; -- end of START GCODE --
[Printy mumbojumbo]
; -- END GCODE --
M140 S0 ; Éteint le lit chauffant
G91 ; Mode position relatif
G1 Z10 X-10 Y-10 E-20 F2400 ; Effectuer un dégagement de la structure et rétracter le fil
M104 S0 ; Éteint la chauffe de la buse
G90 ; Mode position absolue
G28 X0 ; Revenir ?* l'origine 0 en X
G1 Y5 F8400 ; Avancer le plateau au maximum
G1 Z100 F2400 ; Positionner la buse ?* 100mm de hauteur
M18 ; Désactivation de tous les moteurs
M84 ; Arrêt de tous les moteurs
m81 ; Arrêt de l'alimentation ATX
M106 S0 ; Eteint les ventilateurs (ou S0.7 dans certaines implémentations)
M107 ; Extinction des ventilateurs
; -- end of END GCODE --

----------------------------------------------------- Cura Alfawise 20 GCODE -----------------------------------------------------
; -- START GCODE --
M190 S60.000000
M109 S210.000000
;Sliced at: Tue 23-10-2018 13:48:47
;Basic settings: Layer height: 0.15 Walls: 1.2 Fill: 20
;Print time: 2 hours 37 minutes
;Filament used: 8.086m 24.0g
;Filament cost: None
;M190 S60 ;Uncomment to add your own bed temperature line
;M109 S210 ;Uncomment to add your own temperature line
G21 ;metric values
G90 ;absolute positioning
M82 ;set extruder to absolute mode
M107 ;start with the fan off
G28 X0 Y0 ;move X/Y to min endstops
G28 Z0 ;move Z to min endstops
G1 Z15.0 F4800 ;move the platform down 15mm
G92 E0 ;zero the extruded length
G1 F200 E3 ;extrude 3mm of feed stock
G92 E0 ;zero the extruded length again
G1 F4800
;Put printing message on LCD screen
M117 Printing...
; -- end of START GCODE --
[Printy mumbojumbo]
; -- END GCODE --
M104 S0 ;extruder heater off
M140 S0 ;heated bed heater off (if you have it)
G91 ;relative positioning
G1 E-1 F300 ;retract the filament a bit before lifting the nozzle, to release some of the pressure
G1 Z+0.5 E-5 X-20 Y-20 F4800 ;move Z up a bit and retract filament even more
G28 X0 Y0 ;move X/Y to min endstops, so the head is out of the way
M84 ;steppers off
G90 ;absolute positioning
; -- end of END GCODE --

Now, being the sharp witted fellow that I am (meaning I more or less learnt how to read) I started to compare the instructions and tried to understand why one was working and the other two not.
Alas, to no avail.

Would you all be so kind as to help me understand? I'm thinking I can apply this to most printables that failed and make it work, but I'd very much appreciate to understand WHY it works.

Many thanks

[Bikeracer2020]

Problems with filament not sticking to the bed are usually because the nozzle is too far from the bed for the first layer or a bed that needs cleaning.
Suggest you look at some of the numerous videos on Youtube about getting a good first layer and setting the 'Z' offset.

Try just using the start and end G-codes that are in the printer profile.