Dye are pretty good about sorting things like this out, return it to the retailer you got it from and I'm sure that they will do what they can to repair/replace it.
failing that, you can take the assembly out completely, then lift the motor/gearbox out and have a look at it yourself. The on button is held in place by the gearbox so it may have come loose or something?
I had a similar problem, turned out the gearbox wasn't sitting straight in the bottom shell. You need to line up the ridges on the shell with the ridges on the gearbox otherwise it goes in skew.
its strange coz i take the bottom shell out and it works fine Turns on every press. But it back in correctly and it doesnt turn on everytime. could it be a battery issue??