While not specifically with the Radar, I've encountered problems charging some phones on some computers because of variance in the current available over the USB port. Are you charging with the wall charger or with the computer? Also are you using the original cable that came with the phone?
you probably have a faulty charger. You should go to the t-mobile store and try it out with HTC radar charger and if it does not charge then call them and exchange the phone if you are under the warranty period or have extended warranty
well i went to the store about it and none of their chargers would work either, so they jsut sent me a new phone. then after i get it the chargers still dont work but my hubbys works on mine so i im going to tmobile and having them give me a new one that works