Rut Roh ....
I figured it out ..... my fault, its the first time I have done this search, and I forgot to check the number of work units being requested against the time taken to crunch one. For the length of these work units it was a dumb number, I had assumed all was well as the server didnt say - in effect - "go away max is ...." etc.
I will minimise the returns as much as I can, but there will be a fair few abandonned ones ..... I'm sorry ...