The answer's quite simple. The mains charger is rated to give 2 Amps at 5 V. Most generic USB chargers and PC USB ports only give 0.5 Amp - if you are lucky (which is within the USB spec). So there simply isn't enough current to start off a full charge. Worse still, drawing too much current from a cheap adapter could cause the regulator it to overheat and fail, and stuff 12V into your precious phone. I doubt if any of them have over voltage protection.
It's silly to spend $1000 on a phone, then try and save a $10 by using a cheap / borrowed charger. Even more so to continue to try and use it even though it's obvious it doesn't work. Just because the plug fits doesn't mean it's up to the job. Don't risk it. Use something that is specified properly for your device. If you want something more flexible than the manufacturer offerings, Boxwave have a range of high curent chargers (>1A) for this purpose.
I feel ya.. I have just confirmed on 2 different occasions.. Overnight charges.. That the off brand charger that I got for unit.. Is in fact draining the battery and not charging it. Regardless of how much battery is left when placed on that unit.. The next day on both tests.. I couldn't turn the unit on and had to use the original OEM charger.
I will from here forward only use the OEM charger and OEM USB charger.. Not much I can do with the car charger as it does its duty and its never in there more than 7 hours..
just an FYI..