Check the walking belt is tensioned correctly.

Remove power from the mach.

Most machines you should be able to slide your hand under belt, only to about the center of your palm or maybe your thumb.

if walking belt tension is correct. make sure the delt has lube on it. Your owners manual will determine what type (wax or silicone) or manufacturer to use.

When inspecting the deck (laminated board under the walking belt, which it slides on top of), check for wear or damage.

Worn decks and belts should both be replaced at the same time.

Beyond a walking belt area issue, check the drive belt under the motor hood. connecting the drive motor to the front roller. try twisting the belt on its axis. about a 30degree deflection would be standard. If this belt is loose, the front roller is loosing its drive.

Sounds primarily like a belt/deck lube or wear issue.