The UltraFire chargers with those sliding things to contact the batteries are notorious for making cruddy contact with the batteries. You need to poke and twist the battery to get it to go red so that it'll start charging.
I used to have that one (from DealExtreme) but I eventually bought this one instead, and it works great without the fiddling with the sliders:
http://www.dealextreme.com/details.dx/sku.6105