The most likely cause is a lack of tension on the belt.?

I'm not familiar with this particular model so here's a few diagnostic pointers;
It may be that there's a spring-loaded belt tensioner that you've not properly reattatched.?

I could be that there's a (or a couple) of tension-adjustment bolts, around the drive-motor, which can move the motor a little, to take up the belt tension.?

It could also be that the new drive-belt is NOT the correct one for the job.?
If there is no tensioner or adjuster, then the belt will be a 'stretch' type, which is a tiny bit undersized & needs to be gently levered into place on the motor & drum pulleys.?

The old belt should have a part number on the webbing on the outside of it.
Make sure that the new belt has exactly the same part number on it.?