When the sum of an addition operation is blank, which is to say Null, it's usually because one of the value being added is itself Null...Anything plus Null is Null.
But the your example shows that this is not so, in these cases.
I suspect that it must have something to do with the data in that particular 'Bookings,' but I suspect that we'll need to actually work with a copy of your file to figure it out. If you could compile and ZIP it up and attach it to a post, I'm sure someone will be happy to take a look at it.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007