Is anyone else getting this problem when trying to cut an opening in a face? No cut happening! It’s the single biggest bugbear I have with SU. http://bit.ly/200221su
99% of the time this is due to either not being in the same context as the object you want to modify or your walls not being square, so the resulting rectangle is not coplaner with the original. I suspect the second, check out the edges of your inference lock (down arrow) see how the purple is uneven on the bottom edge, I think you are infernce locking to a slightly different plane than the one you want, it indicates something is tweaked with your wall. upload your model file here and you will get a definitive answer.
Thanks endlessfix. I remodelled all the walls and it sorted it out. Thanks for the tip on the wonky bottom edge.