ReOrder List stops application building (sometimes)

I have a small application with a page which has 3 Ajax Control Toolkit reOrder lists.

The application sometimes wont build. Sometimes restarting Visual Studio (2012) sorts the issue, sometimes a pc restart - but these don't always fix the issue.

The error I get (for each reorder list) is :

'ReorderList_Started' is not a member of 'Scheduler.net.BookingScheduler'

I'm using version 4.0 of the framework and the stable latest build of the toolkit

Can anyone point me in the right direction

Thanks
Danny