The cause of poor loading performance from these old cassette players is usually a perishing main drive belt becoming slack as the rubber dries out becoming stretched and inflexible with age.
A "kink" also forms where the belt has sat without moving around the drive pulley for 20 - 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 to the same degree, but can slacken off over time and may start to slip.
These replacement belts are as close as possible to the design and size of the originals but are made from modern, more resilient materials.