It's been seven months with no updates here, so maybe OP got it sorted, but for anyone else fining this thread by search, like I did:
I suddenly encountered the same problem, on a Mac. Everything was working fine, I added a new scene and source during a livestream, and suddenly lost all ability to resize that or any other source. The bounding box is there, the grab handles are extra large, but if I try to click on anything in the Preview window, and the selection is simply cleared with no effect on any sources. The Transform settings have no effect. Neither Preview nor Sources are locked.
But there was a simple fix - resize the OBS window itself (just a tiny bit), which causes the Preview window to reset its zoom, and everything returns to working normally.