Digital cameras are famously demanding on batteries, and the early Fuji ones were notorious. Ni-MH rechargeables are definitely the way to go, but you don't mention the capacity rating of the rechargeables you are using.

NiMH batteries with a capacity of 2700mAH or more are available, and these will obviously last longer per charge than those rated 1600mAH or so. However, all things come to an end, and while Ni-MH cells are supposed to be rechargeable hundreds of times under ideal conditions, in real life they will lose a bit of capacity every time until they become a bit short of puff.