Without seeing it it could be so many things.
The belt could also be too tight. 2 days ago in the forum a user was having the same issue it was because his belts were too tight.
I think I have it in the directions as don’t actually tighten any of the bolts until you run it a bit.
I was also just looking through thingiverse and the 5th set of belt tightening parts just popped up using 8mm bolts. I can promise you right now those are a bad option. But if you read the comments it is from a person that hasn’t actually used his machine yet.
So many “improved” parts are form people that have not even run there machine 1 time. Tons of builds are getting made with lots of extra parts and add ons it is getting hard to trouble shoot.
Please post a picture of your machine so I know what we are working with here.