Firstly that belt is toast judging by the crack in one of the photos.
Secondly it is either the wrong belt or is severely worn.
The proper belt , which is expensive is a wrapped kevlar fibre belt.
That belt appears to be a plain unwrapped el cheapo ( cause I know better than all those thieving rat bags who force me to buy those overpriced funny colour belts when a regular one at 1/4 the price will work fine ) belt.
If it is a plain unwrapped belt then you problem , or a large part of it, lies there.
Unwrapped belts are designed NOT TO SLIP .
They quickly get hot & sticky just like your car tyres.
Wrapped belts are designed to slip for the purpose of clutching.
Your belt should be # 21546076 which is an A 90.375" belt.
Plain belts are made in whole inch sizes only so pretty sure you will find it is a plain A90 belt.
If Mr Half Witt had bought a wrapped Kevlar A 90 he may have gotten away with it
But using a plain belt that is also 3/8" too short is asking for trouble.

