Why will my newly installed battery not charge/turn on?

I've recently purchased a battery from this site and attempted to install it into my ndsi xl. To my eye, the original dying battery and the new one are identical (except the new one has a higher mAh, but that doesn't strike me as a problem) and I'm rather sure I installed it correctly (it fits in the slot, the electrodes are touching, etc) but when I attempt to charge it, the charging light flashes periodically and the device won't turn on. When I replace the old battery, it charges and turns on just fine (though, still, dying battery that doesn't hold charge well). I apologize if the answer is obvious; I am nowhere near an expert and thus have likely not done any of the basic troubleshooting steps that you are probably thinking of. Thank you!