Not normal. You mentioned the bike is new so the first thing you should do is take it to the shop you bought it from and let them inspect it. Anything you do to correct it and fails, the shop can blame you for the problem and not honor any warranty leaving you with the cost of repairs. Having said all that, if not under warranty it sounds like a case of lack of, or sticky lube in the pawls of the cassette hub so it doesn't always engage but could be something else too.