could she be overtired from getting to nap too late in the am? i know that both my boys would go back to sleep only 1.5 hours after wake up time. that would make your time 7:30am. maybe by the afternoon she is just too tuckered out to sleep.
also is she taking 3 naps or 2? not sure when they drop the 3rd nap (the middle one). but our guy sleeps from 730-9 then 1030-1130 and then finally from 1-3. then off to bed at 530pm until 6am. these are all approximate times. the evenings are definitely sooo hard for our guy. i swear he would go down for the night at 5pm if i let him.
not sure any of this helps. i have read much on sleep and still feel like such a novice. the most valuable advice was weissbluth, "sleep begets sleep". in other words the more they sleep, the more they will sleep. we have found this to be true with both of our boys.