Im half way through a level and all of a sudden I cannot build rooms? It allows you to click lets say the scanner room , and the writing extend blueprint until your happy with it pops up and all you can do is ujst extend the blueprint , I cant confirm the size or anything.... ideas would be most welcome

You might be low on cash? Or is there a staff member wandering around through your blueprint area, or a hidden piece of furniture stopping the blueprint area being a valid room size?

I'm having the same problem - suddenly, in the middle of a level, I cannot build new rooms. I have the money to build the room, there are no staff in the blueprint area, and there is no hidden furniture. Also, the doctor on the right hand side keeps popping up to say that someone is stuck somewhere in the hospital, but that isn't the case. I also noticed that when I bought a new building, that new building has a hole where the door should be... Any suggestions about how to fix this?

I think I have seen this problem. You start to build the room, you lay down the blueprint but you can't complete the room because a message pops up that says this room blocks other rooms. Right;

The problem is that there is a room that is badly build and no one can access it. (It's not necessarily the room you are currently building). I found out that I had a toilet room where the door was facing the corner and it had other rooms around and no one could get to the toilet door because there were walls all around and no path. (see image)
The game will allow you to do this, but when you try to build the next room you will get stuck and you can't continue the game.

So reload from an earlier point and search for rooms that might obstruct other rooms, or face the corner (like I did),
correct them and continue building.