That type of charger uses a low current to charge batteries, which means it takes about 8-10 hours at minimum to fill up a couple of 2500+ mAh batteries. Ideally, you leave them charging over night (14h+). 6 hours is not enough to charge good batteries.
In addition, older Canon camera models have the low voltage threshold too low, they tend to say low battery and turn themselves off too soon, at around 1.28-1.3v per battery. Rechargeable batteries are around 1.35v when new and around 1.2-1.25v when completely discharged - basically, the Canon camera will ask you for new batteries even if there's 20-40% of energy still there, in the rechargeable batteries.
The camera also pulls a lot of current (1-2A) for a few seconds when it charges the flash, and that sudden draw can pull the voltage on the batteries down for a few seconds and then batteries recover, but that's enough for the camera to think the batteries are discharged. The lcd display on my A580 alone uses a constant 150-200mA and when focusing or preparing to take a picture (snap picture button half-pressed) the current draw goes up to 3-400mA - i tested it by connecting the digital camera to a linear power supply.
If you don't believe me, try with regular AA alkaline batteries.. with 1.65v when new, the camera will take quite a lot more pictures before saying the batteries are dead.