This sounds like the dreaded 'memory effect'.
With the older type of rechargeable batteries (nickel cadmium), it was widely known that it was necessary to fully discharge the batteries before recharging them. Otherwise, the 'memory effect' could come into play. For example, if you had a set of batteries which would give you 200 pictures and you used them until they were 'dead', recharging them would make them fit for another 200 pictures. However if you only took 50 pictures and recharged them, the batteries might only be fit to provide a further 50 shots or so. After just a few times of giving 'top up' charges, the batteries would be virtually useless. (Good quality battery chargers were designed to totally discharge the batteries before commencing the charging process, to overcome this problem).
When the newer rechargeable batteries (nickel metal hydride) were introduced, they were more expensive than the older type but they sold well largely upon claims that they didn't display the memory effect. So I bought some and, assured by the claims that I could top them up before they were fully discharged, always topped them up after just a few pictures. Very soon, I experienced exactly the same problems which you're having, i.e. the batteries wouldn't hold a charge.
Concerned about this, I e-mailed the manufacturers (Duracell). The reply stated that, while the effect is not always as great, nickel metal hydride batteries also suffer from the memory effect and should only be charged after, or close to, exhaustion. (Alternatively, of course, a battery charger which first discharges the batteries could be used).
Unfortunately, once batteries have acquired their 'memory', it seems that there's little that can be done to bring them back to full use. You'll need to buy some new rechargeable cells and then ensure that you don't recharge them until they're nearly, or fully, exhausted.
Chris