First things first, if it's still under warranty (you don't mention model year) I would check with the dealer to make sure there's nothing wrong with it (assuming you purchased it new from a kymco dealer). I can't find anything indicating that the Sento's have a speed limiter on them that would keep them to moped speeds (similar to the older Honda Metropolitan 2). There should be no reason that your scoot won't do at least 35.
It could be something simple that should be covered under the warranty.
