Trouble submerging or swimming lopsided are two key indicators of illness in a turtle. Other indicators are lethargy, poor appetite, breathing with the mouth open, swollen eyes. If you're seeing some of these things (you don't have to see them all) then it may be your turtle is getting sick, in which case a quick visit to a vet is going to be necessary to save him/her.
Seeing as the tank is quite small relative to the size of the turtle, it's possible that you may be having ammonia and/or nitrite buildups which are making your turtle not want to submerge. Do you ever test water quality? There are kits for this that you can get in the fish section of the pet store. In a healthy tank, the readings for ammonia and nitrite should be zero. If you test and find ammonia and/or nitrite, then you can check back on ways to reduce it down to zero. (The first suggestion is going to be to upgrade to a larger tank!)
One thing you can do right away that will help water quality and also make your turtle very grateful is to put a lot more water in that tank. Turtles are aquatic and love to swim and dive. Here's a picture of a 29 gallon tank which is filled up pretty well:
http://www.redearslider.com/forum/viewt ... =gallon%2A
You have be careful not to put so much water in the tank that the turtle can climb out and escape. They are great escape artists! So you probably would not be able to fill your tank as high as this one because your turtle is bigger.