Shrimp make a good treat but there isn't much nutrition in them for a turtle. What brand of pellet have you tried? Turtles can be picky about their pellets, I have one who'll eat Mazuri but the other won't. Both will eat Reptomin, but neither will eat Wardley's, neither will eat Pretty Pets, Nutrifin, or Rep-Cal.
Sometimes you just have to keep trying different pellets until you find one they'll eat.
Rotating foods, not feeding them the same meal twice in a row is better for them anyway & it can help to keep them from becoming finicky eaters as they grow up.

PHRatz