Hmm. All the levels available (including the final level)have glowing frame scores. I have all the other hidden levels with glowing frame scores. Hope it's not a glitch...
I'm assuming it's a red gem level - there are 8 secret levels right? And I have completed the game, and I have completed the other 7 gem levels. There is still 1 secret level greyed out on the amulets screen.
The secret red gem level is located at the top of the map, directly above the second Epic level. Check the levels around that area closely to make sure they are all glowing. If they are, try replaying and rescoring just the 4 or 5 levels right in that area, and see what happens.
if you have glowing on all levels in the game (including epic) then it should be there. I have beaten every level, including that one. It may be a glitch, I don't know.
It IS a glitch. Happend to me when I unlocked a secret level and saved, quit and when I loaded it wasn't available anymore. Can't even unlock it again. Hurts.