How many and what sort of sources do you have listed?
I ask because some sources are somewhat memory intensive and it takes some time for data to be written into and then read from RAM.
I always have to allow OBS some time when I start it up to "shake itself out"; if I try to do anything within or upon the OBS window (and I mean anything - such as resizing or dragging the window on the desktop) I am going to get a not responding message.
After a small amount of elapsed time, this issue self-resolves.