macally power adapter issue

I destroyed the original power adapter for my iBook G4 and bought a macally adapter because it was cheaper. It worked fine for almost a year, then stopped charging unexpectedly. The blue indicator light still comes on, and if I plug it into my roommate's iBook, it works. If I then plug it into my iBook, it also works, until I unplug it again. It seems that if I first plug it into her iBook, then mine, it will work, but once it comes unplugged from mine, it won't work again. I'm wondering if I could just buy another macally charger, or if I need to buy an apple charger (her original charger works fine in my iBook).