If your Zumo charges from the wall socket or PC then that proves the battery connections and the battery itself are OK(This uses the mini USB connector).
Most problems will be either with:-
1. All electrical cables and connectors it best to remake all the connections and test for continuity good earthing and power supply +ve to +ve and -ve to -ve.
2. Check for 12 volt supply this may sound strange but a bad connetion will lower the voltage and your Zumo will never charge. Check for a good earthing, Check your fuses.
3, Check your wiring loom for cuts and twisting.
4: Clean all the multi-pin connections on the back of the Zumo and the cradle pins and that there is no damage to connections or pins.
5. Find someone who has a similar cradle and see it will charge if it does then the cradle or pins are at fault. try going into Touratech they are very helpful and can test it in one of their bikes cradles
6: Open up the cradle and check the internal connections for broken or dry soldered joints.
7.Its a process of ilimination go step by step inch by inch.
8:When the Zumo is in the cradle try pushing the zumo down into the cradle to push the connectors closer together.
9. When you close the back door on the Zumo after charging on the wall socket is it closing fully it must be a tight seal as it opperates an internal switch inside.. There should be a plastic hook on the back door which engages inside the zumo onto a switch is it broken?

Thats all I can think off, good luck.