I ran into this problem quite a bit during my first few months.
- calibration of the extruder.
- x,y,z calibration as tight as you can make it.
- make sure nothing is loose, my x-carriage had cracked and was moving quite a bit. that was my bingo but all the other factors were also off at one time or another.

I made sure I calibrated with the firmware config file, left my slicer settings to default as much as possible. I use slicer settings for specific parts, little tweaks. Firmware is used per filament roll (I keep a file for each roll, often I can go by batch, 1 maker, 1 color, 1 type of filament (pla,abs,laywoo, nylon). I put a sticker on the roll, write down my parameters, temp, config file name and I keep them, they become my starting point when I buy the same again (each bloody roll tends to have it's own tweaks and cheap filament, I calibrate 3-4 times per roll and numbers tend to vary enough to cause small issues)

And now I print like a pro, a king, champion of the ring ;-)

Hope this helps.

~ Doing my thing with a Prussia RepRap i2 built from scratch, pain and sweat. I reprinted all my own parts, the kit had shady workmanship, workable but real crooked. ~

M.