Since it worked well at the beginning, I don’t think it is your belts, but I would like you to look at each motor and make sure that each motor flat has a set screw tight on that flat, and report back.
What a change, I slack my belt to access de pulley and all of them were lose and was remove whit no difficulty only whit too fingers. Tighten the 4 pulley, install the belt again et make a test. Wow. The result was astonishing, the router pass too time on each word and no mistake.
Thank you for your help. is this problem can come back again?