I ran afoul of the mongrelizing of cucurbits that Jal mentions. Last year I had a bunch of really nice Prizewinner pumplins, some well over 100 lbs. I saved the seed from the biggest to try this year. I also had two other pumpkin varieties in the patch as well a several types of gourds.
I had a crop of the most diverse bunch of mongrels you could imagine. Not got really big, some were orange with dark green stripes and blotches, not a one could be identified as a Prizewinner.
I read how to hand pollinate just recently so mext year I am going to try getting true prizewinners to save seed from and may try crossing them with Big Max to see what I get from the marriage.