The cause of poor loading performance from the Datassette units is often a perishing drive belt becoming slack, or more commonly kinked where it has sat without moving around the drive pulley for 30 years or so. This kink then becomes a permanent feature of the belt causing it to jump and slip.
The tape counter belts don't usually snap or degrade, but can slacken off over time and may start to slip.
These square section belts are of identical design and size to the originals but made from modern, more resilient materials for the version of the early VIC C2N "shoe box" style unit which was made in Taiwan.
It is important to identify the exact model and where your Datassette was manufactured as the belt sizes used are different - see the photographs and check the bottom of your unit.