Aerox belt slipping

As some are aware I have upgraded my aerox 2014 with sports exhaust, polini variator and Kevlar belt. I had a few issues but all sorted. What I'm left with now is the belt is slipping for the first 30 seconds of riding. I can hear it slipping, so what could be the cause.

Once the bike is past the first 30 seconds there's no slipping what so ever and it goes pretty well to 55+mph with a 16 stone lump on it!

 

My belt took around 150 miles to bed in just give it a bit of time until the preservative wares off.

 

Already done 200 miles + I like it's a common fault with Kevlar belts when cold plus my weight don't help the poor little thing!