more importantly, what are you plugging the charger into?
if a wall wart usb power supply, make sure it puts out at least 5v @ 750ma ... if a computer, make sure you use the front panel USB if available - they put out more power generally.
i plug my ego chargers (normal 4.2v 420ma) into phone chargers that put out 5v @ 750ma - or my pc which will put out 5v @ 1a ... never have a problem with them.
another thought - how tight are you screwing them down? you could be mashing the center terminal sideways ... i just tighten mine until the button blinks and leave it like that. one of my chargers, that takes like 10 turns, the other, about 3 ... and i always screw the battery on with it plugged in just for that reason.
as long as the battery or the ring with the button isn't getting hot to the touch, i wouldn't worry too much.
yet another thought - did you make sure the terminal / threads were clean? juice can gunk those up quick if you are not careful.