Tired of clicking back into project window in order to play

Just upgraded to a new comp with win 7. I’m using Cubase 5.5 64bit. When I open the window for a vst or vsti’s interface and I start tweaking knobs, instead of being able to just hit the space bar to play my track, I have to click on some part of the project. As soon as I click on anything in the vst/i GUI, I can’t play from the space bar. Super annoying and time consuming. I don’t remember how to fix this. Thanks for any help.