When I first got my turtles I was feeding them pellets everyday
When I switched to lettuce, they wouldn't go for it, I sprinkled krill on it and they went for it, until the krill was gone. Finally I just threw the lettuce in and figured they would eventually get hungry and their instincts would kick in and eat it and see how satisfying it will be for them. When you're hungry, you'd be surprised how "pickiness" pretty much just flies out the window..
I threw 2 huge pieces in their tank and walked away, by the end of the day, it was gone. Ever since they are really good about eating lettuce.
I think the way I introduced it really helped though just don't make it a habit of the lettuce having to be sprinkled with krill, it's a very high protien.