L.H.
My one son was a thumb sucker, He even sucked a whole into his nail. We have him know who is 10 pretty much without his thumb, but sometimes we catch him. My brother on the other hand. Was sucking his thumb all the way till the age28 and that was during t.v and really tired.
Keep working with it during the day and the rest of the time will work out. Also there use to be a nail polish out there, Also try that if she goes so long with out sucking her thumb that maybe see about have a little girl time and do her nails or something like that.