See this thread: http://www.primegrid.com/forum_thread.php?id=5157
I was requesting 25 WUs at a time and returning just 24 of them due to sockets error in the PRPNet code. You can see if this is happening to you by looking at the Pending Test list.
Nothing to do with expiry times. Developer of PRPNet, rogue, recommends reducing it to 20 workunits max so that you don't lose any work.
I am requesting 100 WUs at a time but I haven't seen a single error yet.