Without knowing how your event is coded, I'm guessing it has something to do with fee ordering. I know that has bitten me at least once since I've upgraded from 2.6.x. I think there's a video on fee ordering it will probably explain what needs to be done a whole lot better than I can.
The fee ordering is a powerful feature, but it does require a bit of getting used to if you are coming from 2.6. One of our other admins had no problem figuring it out, but he just started on 2.7...
Hi jstephens, Really appreciate you trying to help there.... I thought you'd solved it for me because I'd never used that 'Fee Ordering'.... but sadly no - Same behaviour..... I have a ticket in so I'll await DTH taking a look.... maybe it's something specific to me.