Z-axis steppers and bed alignment problems
I assume you did everything according to the instructions but here is a checklist of what could be possibly wrong:


Friction - check if you can rotate/move parts without lot of resistance
Screws - check if screws on couplings are tight and they don't slide over a shaft or thread
Stepsticks - check if they are cooled properly and similar (as there are two of them)
Carriage nuts on threads - check if they do not slide out of their nests while [the x-axis] carriage goes up
Filament - check if filament unrolls without resistance which can eventually cause [the x-axis] carriage to hang.


IMO #2 and #3 are the most possible cause