2100mAh should last a while if you got a full charge into them, typical quality Alkaline's will be more like 2800mAh though so don't expect them to last quite as long.
If the rechargeables are dropped or damaged then don't expect anywhere near the 2100mAh figure. If one cell is not performing then it'll drag the other down with it. Just because they are new doesn't mean they haven't been kicked about in the shop you got them from.
If you have a set of four try swapping some cells around just to determine of you've got a weak one.