Motorola Droid 2 :: Can't View Photos And Videos Taken With It

Sometimes when I take pictures and/or videos with my phone, I can't see the picture or video on my phone. I take a photo. The phone acts like it took a photo. I see a thumbnail initially. When I try to view it again, there is just an icon and the photo is gone. It will say it isn't viewable or a video isn't playable. Can't view them if I transfer them to my computer either. I don't understand what the problem is because sometimes it works just fine. I can still see old photos or videos. The formats are exactly the same as the old photos. I didn't change the settings.