Is there a red line above the Clips on the Timeline? If so, hit Enter and check playback.
Next, can you give us some more details on the those HD Flip files and also your computer? This ARTICLE will give you tips on the types of info that would be useful.
In most case, though, Flip video simply won't work in Premiere Elements and will need to be converted to a more standard format, as detailed in the FAQs to the right of this forum.