This happens way too often. I try to drag a video file into my timeline from a finder window in Mac and it bounces back to the finder, refusing to go onto the timeline. I'm then forced to import it into the project and THEN drag it into the timeline. It happens on projects where I was able to drag video and/or audio clips onto my timeline and then it just randomly stops. I'm tired of it and it's happened over the course of at least 2 years. Fix this. It's a blocker.