Sheildon usually evolves at level 30 - so if you are sure that it does not have an Everstone equipped, then the reason is one of the following:
- Your game is hacked or glitched
- Your Sheildon has an Everstone equipped (just double check)
- You have used a pirated version of the game
- You have used an Action Replay card
or, you used a ROM game card such as R4.
If none of those circumstances are evident, then the reason your Sheildon won't evolve is uxplainable, unless you go really technical into things.