iMovie and Flip Ultra HD video camera...

...I have a Flip Ultra HD video camera that's maybe 5 years old, still works well. I have Maverick on my 20" Intel iMac and 13" Intel MacBook Pro. iMovie will import the video clips and allow me to stitch them together, even add transitions between clips. But when I try to play the finished movie, I hear audio, but the video will not play past the transitions...any help will be greatly appreciated! Is this a compatibility issue? I've used the Flip camera many many times in the past with iMovie and made great movies with no trouble...what's changed? ( besides my update from Leopard, Lion and Maverick...) Thank you in advance.