What do you mean "on full screen"? Most people WANT the video to play full screen, what exactly do you want the video to look like? ANd what do you mean the videos "arent playing thorugh"?

 

Welcome to the forums, hemptree.

Video issues can sometimes be remedied by going straight to the video app in menu > settings > applications > manage applications > menu > sort by "all" and then clearing the cache and data (and defaults if that is available).

Then reboot.

Also, check to see if your video configuration is set at highest quality, sometimes that will slow things down and cause lack of playing through and other screen issues such as jerkiness, etc.