Thanks Allocator, point taken, although with the traffic over my location I do actually find it easier to select individual aircraft rather than wildcard entries. Also with the custom sound file that I am using, an american railway crossing bell, it gets a bit noisy when using wildcards!
Further to my original post, I have just received an alert for the last aircraft on the list, so maybe the problem is intermitent.
Maybe I should reduce the number of a/c in the list to certain regns and just be more alert myself.
Thanks for the reply, Dave.