This was my problem also. My Mugen battery fit quite snugly so I didn't think a shim would make any difference. But when a replacement failed the same way, I added a couple of thicknesses of masking tape to the end of the battery. It now charges and works OK. I'll have to see if the fix is reliable. The photo below shows how the Mugen contacts don't extend as far as the Nokia BP-4L or the other third-party battery. And the contacts look less sturdy.