This is well know problem with the TOMTOM rider version 1. The Rider 2 has a different cradle and wet connect.
The problem with the Rider 1 is the 4 little brass pins are supposed to connect onto the 4 little brass plates on the back of the unit. I have had 2 cradles now for my Rider 1 and none of them line up properly, thus affecting the connection. The pins also tend to wear out on the tip. All-in-all not the best design.