I assume you checked the orientation of the batteries in the carrier? I mention this as I have an L7 that takes three AAAs and one of the batteries sits with the positive contact against the spring loaded side. I'm not familiar with the cartridge in the T7, I know it takes four AAAs, perhaps it shares the same type of layout. If you're popping the batteries in with the assumption that all of the negative terminals should be on the springs, that may be your problem. Each battery slot on the L7 is marked for polarity but they can be easily overlooked, especially if you're used to having springs being the negative contact.