WEB Service Drop Out

Hi

I am having continuous problems with a web service we have developed.

The web service works like this

Customer id and filename passed to webservice
process runs - it moves and renames the file it also shells out a command line as well

however when more than one call gets made to the web service the prior one ends (this happens alot at the  part where the shell is happinging)

i really need help with this - it is causing all sorts of problems

Thanks

Matt