Sorry, what do you mean you couldn't get yam seeds anymore?
Is there another way to get yam seeds besides just waiting for them to unlock?
If you save the day before the crop festival, then sleep, you can check what the prizes will be. If you don't see what you want, you can reload and try again. This way you can get Yam seeds as a prize in summer year 1.
However, if you save that day, then the prizes will remain set.
"To hell with your spoiled baby! Zoidberg needs those shoes!"