The trickle charge may be the problem, i.e. if the charger does not apply one and drains the cells slowly instead. Besides, it is not recommended to leave your cells to trickle charge for long periods. What I do is leave them for no more than several hours, depending on the trickle charge current of the charger. This serves more as a top-off charge than a maintance charge. If the cells have been sitting for more than a couple of weeks, I just top them off. Healty NiMH cells, at normal room temperature,shoud not lose more than 20% of their capacity in a two weeks period, so even without the top-off charge they should be OK.
In any case, if you doubt that your charger is deffective and it's still waranty covered send it for check/repair.