Evolution, Exchange and Windows Mobile Calender

I've found something odd going on with calender sync. I have Evolution
2.10.1 running on Ubuntu and connected to exchange server.

If I create a new entry in my exchange calender via evolution client it
syncs fine with exchange and can be viewed via outlook web access or
outlook.

However, the calender item does not then sync with my O2 Mini S phone
which is running Windows Mobile 5 and connects to exchange via direct
push. If I edit the calender item in exchange it then syncs with both
evolution and my phone.

Anyone else come across this and do you know hoe to correct it?

Regards
Chris