I assume you did everything according to instruction but here is a checklist of what could be possibly wrong.

 1. friction - check if you can rotate/move parts without lot of resistance
 2. screws - check if screws on couplings are tight and they don't slide over a shaft or thread
 2. stepsticks - check if they are cooled properly and similar (as there are two of them)
 3. caret nuts on threads - chcek if they do not slide out of their nests while caret goes up
 4. filamet - check if filamet unrolls without resistance which can eventually hang caret
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IMO #2 and #3 are the most possible cause

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/4Hd33.png