Well, first of all, those 7dayshop batteries have a tendancy to be pants. I seem to recall reading somewhere on this forum some people saying they were okay, but in my experience, they were rubbish.
They drain rather rapidly in a GP2X, but I am reliably informed by my dad (who I gave them to when I switched to some Energisers) that they work a treat in a telly remote.
But second thing you should be aware of is that a new set of batteries will take several charges to reach maximum capacity. How you treat them in the first few days is crucial. Charge them to the max, and then drain them completely, several times over. You should notice that it takes progressively longer to drain them, until they start reaching the desired 3-5 hour mark (depending on what you're running, of course).
This is a general rule of thumb with rechargeables, though, and probably doesn't apply to your specific set, which I am sorry to say will remain rubbish for all time.
