I'll play Devil's advocate here and ask the question; Are you possibly over training Scooby in preperation for his test?
Drop the retrieving execises for a few days, maybe a week and see if his desire returns. He may be a little dull at this point, and the "game" is becoming less fun. He's blinking at the fetch, and you definitely don't want him to start blinking at birds, so it may be better to back down on the retrieve before it becomes an issue in other areas.
You definitely don't have the time to Force Train him to retrieve between now and the test, so backing off, or introducing a new dummy, may get him back on track.
When I get frustrated with my dogs, it's a clear signal to me that I am missing something and need to back myself down and rethink my approach. It's an odds on bet that I am getting ready to make a mistake which will cost me down the line when I have to reapproach an issue. If you find yourself getting frustrated, find something to end the session on a positve note with and quit for the time being.