Have a read of this thread on a photography forum I use. Standard NiMH cells do self discharge, though it should take a few months, not a few weeks. It could be that using crappy chargers has damaged the cells.
Eneloop cells are much better, they don't really self discharge. Even with the lower capacity i'm buying them instead of higher capacity cells that self discharge.
If you want to send me your cells I can run them through the C9000 and see if they recover. PM me for my address.