I take it this is the latest clamp which has 4 silicon buffers, a c clamp and the magnet has one half with 4 holes in it to connect the clamp with the buffers? Here are the possibilities that come to mind.
1) While it is close, they are not quite symmetrical, the holes are in a rectangular pattern so if the magnet is oriented one way and the clamp another (offset 90 degrees), the buffers are stretching to fit and not making secure contact.
2) The silicon rubber of the buffers will dissolve in acid, if a strong acid was ever used for cleaning they may have lost some diameter and been damaged so they no longer hold.
3) The silicon buffers are not being inserted to the point with the groove so they lock in, most likely due to the first cause.