Missing frames in timeline.

I've been using photoshop to animate the past while but I keep running into the same problem. For some reason, and I dont know what makes it happen, sometimes I'll go into my file and everything will be fine and i have a smooth animation. But other times for apparently no reason some frames just simply wont appear at all but when I render the video they are there. But my problem is it makes the frames in question uneditable. Please help me I've been trying to fix this problem for weeks now.