I've noticed the gearbox refusing to change up recently. Fist of all, it was just a case of the revs rising to 2500rpm before the change up when going down a slight hill in the morning. Today, accelerating gently from stopped at lights on a dual carriageway, the rpm went to 4000rpm without changing. I then close manual and was able to reduce the rpm that way. Is this unusual? What could be the cause?