Threads printing successfully, but not fitting successfully (Makerbot Z18)

[Mantroll]

Hey - could really do with some help 3D printing threadsThe print quality seems to be pretty good, but when I go to screw the parts together, I get large levels of resistance once the bolt reaches the other end of the nut, and then it won't go in any further. Please see pictures and details below:

• Printer: Makerbot Z18
• Extruder: Makerbot Smart Extruder
• Slicer: Makerbot Slicer
• Filament: Makerbot standard PLA
• Source: Threads generate from Fusion 360 and exported as STL
• Thread: M20 x2.5 6g
• Both the bolt and the nut were printed vertically

I even increased the tolerance by offsetting the faces on the threaded bolt by 0.2mm. I realise I could buy an M20 tap and die, but am I right in thinking I shouldn't be having an issue with this printer/thread size. Any help would be greatly appreciated... I'm lost and confused!