Threads printing successfully, but not fitting successfully

[Mantroll]

Hi there,Slightly confused as to why my threads aren't fitting together properly. They print fairly well, but when I try to screw the nut onto the bolt, it goes as far as the top of the nut, but then encounters massive resistance and won't go much further.I even added a 0.2 offset clearance on the thread faces. Settings: Printer - Makerbot Z18 Filament - Makerbot PLA Layer Height - 0.1mm Source - model generated in Fusion 360 and exported Threads generated from in built thread tool in Fusion 360 Thread - M20 x 2.5 6g (bolt modified with a clearance offset of 0.2mm)Extruder - Makerbot Smart Extruder Extruder temp - 210 I realise I could buy an M20 tap and die, but am I right in assuming that this printer shouldn't have problems printing an M20 thread successfully?The only thing I can think it might be is I can't quite remember if I chose medium or high refinement, when exporting the STL...