Like santzapaul said, the rule is one issue per lifetime. If denied, you're allowed to apply again. I lost a co-op job and then requested a certificate from my school as I had the required credits. The school issued a certificate, but since I earned those credits over 180 days ago my certificate was dated over that limit. I switched schools to complete the requirements for a diploma and applied as soon as the transcripts arrived, letting me get approved this time.
My completion letter said the diploma course was a two year program despite me completing it in less due to credit transfer, so I'm assuming that's why my PGWP was issued for three years. I did three semesters at my current school with the final semester part-time.