Right, the nets must be named and classed correctly otherwise you can't use P, I at all.
Also, the rules are consistent between layers, right? Obviously, you can't route pairs on layers they're prohibited on...
Interactive routing is weird, sometimes you can hit L and it just places vias wherever (at current follow/look-ahead position, or last solid traces), sometimes it whines and gives you nothing, in which case you have to undo the L (hit it a few more times) and press 2 instead to lay down some vias. And then you might still not be able to switch layers and have to cancel, select the vias and resume from the desired layer.
Is that what's happening, or have you been through that and it's still not working?
Tim