You might have an air lock in your heater matrix, one thing to check is when the engine is nice and toasty feel the pipes going in and out of the matrix to see if they are warm, if only one is it looks like water isn't getting through it and you'll need to bleed the system.
Another thing to check is that both the control wires going into the heater box are hooked up properly, as apparently theres a little cog that can fall off. Easy way to reset them is to put both to the far right, move the flaps in the right position and hook it back up.
