C. - My daughter turned 6 in July and still doesn't stay dry most nights, although, like your daughter, she has been fully potty-trained since around 2.5 years old and has no problem staying dry during naps. About a year ago I tried a lot of different things - too many to list, and anyway, they didn't solve the problem, at least not on any long-term basis. For example, I could wake her up in the middle of the night to use the bathroom, but that doesn't create bladder control for her - it only helps keep her dry because of my intervention, plus it disrupts her sleep, which I don't want to do. So she still wears a diaper at night, which she really doesn't want to do - but it's the only thing that makes sense until she is able to stay dry on her own. I know it will eventually happen, and, in the meantime, I don't want to pressure her or make her feel like she's doing something wrong when I don't believe that it's anything within her control. I know she wants to stay dry through the night - she is very happy and proud when she does - so I will just let nature take its course and hope that it happens sooner rather than later. Good luck to your daughter as well.
P.S. I should mention that I also have a son your daughter's age who was fully potty-trained by 3, but he hasn't had a dry night in his life. At least I know with him not to bother doing everything I tried with my daughter when she was 5. :) I'll take the same approach with my youngest child (20 months old) as well since I fully expect that he will be like his siblings and not be able to stay dry through the night until a relatively older age than most kids. There must be something hereditary about my kids not having whatever gene controls all that!