Why has my iMac stopped playing video clips?

A few weeks ago my iMac stopped playing emailed video clips, such as Utube and others. My MacBook Air continues to open and play the same ones. I installed an app recommended by a Support Community responder, which worked for only a few, and now it doesn't open any. I joined Utube and went through its trouble shooting to no avail. Interestingly when I move the clips from Safari to the desktop and right click to "open with RealPlayer", they open and play (but this is an unnecessary pain). Thanks for any suggestions.