Hey Dymo,
It sounds to me like your old kite sat a little further back in the wind window than the new one you were testing.
Most likely the problem is that you aren't used to a kite that flies a bit faster and hunts to the edge of the wind window when you tension the lines. 5th line kites with too much tension on the 5th will feel really gutless and will do what you are talking about as can kites with a small diameter leading edge and a sleek wing profile.
If you want to continue with the new kite, check to see that you are trimmed right (this will take the eye of a seasoned kiter to check for you) and if it is OK. You may have to adjust your riding style to gain the most out of the new kite.
The new Airush Vapor is like this. Very little static pull, but once you start riding faster it has enourmous power. It is designed to be flown fast which requires you to ride at higher speeds than you are used to.
Just ease off your heel pressure, let the board build up speed a bit and if the power returns, there's your answer.
If you like to ride slow with plenty of power at lower speeds, then you may need to try another kite, one with a more generous wing profile, larger leading edge and lower aspect ratio.
I hope this helps, if not, give me a call or a PM and I will discuss it with you and try and sort you out.
Cheers and good winds,