Batteries dying! Please help!

[Sodom]

Hello guyz. I need some help about batteries. My batt packs do not charge up to the full amount any more. Can only get 2700mah for my GP3300. And i have 6 packs and all of them are the same. All my packs are GP batteries and they are suppose to be robust right? Got this info from my hobby shop. I have these not more than a year but i dont used them weekly. Twice or once a month only. I discharge them using Spintec batt. manager and charge them using Novak Millenium pro. When not using i will half charge and store them. So is there anything wrong with these steps or are there better steps to maintain batteries? And is there any way that i can get back those batts fully charged again? Btw i tried slow charging 1 pack using 0.5v but it gets worse. Only 2000mah. Any one ideas? Thanks