Actually, Troj is wrong about one point. When it says immobilized for 0 turns, they can move that turn. So, Tremor only works for 1 turn, not 2. Freeze works for 2 turns.
I've reported this bug before. I have noticed that army size seems to be a factor, as I've had Tremor hold an Ashwake Dragon in place for 2 turns, but a full army never more than 1. I've seen this bug since I started playing FE back in January.
Freeze has a similar problem, but it actually shows immobilized for 3, 2, and 1 turns. The problem there is that, when it gets to 1, they can move again, having only been immobilized for 2 turns so far.