The cause of poor loading performance from the Commodore 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.

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 1530 C2N unit which was made in Taiwan.