Slightly unfasten the 4 screws on the sides of the carriage, without removing them. This will give the ball bearings inside a bit more room to adjust their position.

Slide the carriage back and forth so all parts in the assembly settle in the best position.

Fasten the carriage screws again.

Do not apply pressure on the carriage when you're tightening.

Do not tighten too much.

Tap your carriage very lightly using a mallet (soft hammer). Tap on every face of the carriage, and on every corner of each face. This is to make sure that any stubborn ball bearing falls into the right position.

Test the smoothness of your carriage again.

If the problem persists even after a couple of tries, jump to the next step.