As Tsubakai said, the quality of your water is very important. If you take some water down to an aquarium shop to have it tested, they will be able to tell you what may be wrong. The pH is the most important in this case, as all the calcium, UV, and Vitamin D3 supplements aren't going to do a thing if your water is too acidic (low pH). Turtles can be quite messy, and if food particles, faecal matter and any other biological material builds up in the tank, it will lower your pH, build up ammonia levels, which, with further breakdowns, will raise your nitrate and nitrite levels. It is easy to let it get to the point where beneficial bacterias in the water cannot keep these levels in check.
In this case, a couple of water changes done perhaps once a week, will help to replace bad water with fresh water with beneficial bacteria. With these water changes, a product called Cycle will help to put a bit of extra bacteria in your water after every water change.
Bloodworms are a great treat for turtles, but should be used as just that, a treat. They aren't a very nutritional food for a turtle, sort of like maccas for people! Crickets, shrimp and small fish are some of the great more natural, healthier foods especially for young and growing turtles.
After the food and water quality have been improved, you should start to see some improvement in your turtle, although it will be a relatively slow process.
