Brocolli seed germination

I planted some bocollit seeds around three weeks ago, but they have not germinated. They were new seeds for this year. I planted them in a large container, and because I was going to be gone for a week, and could not water them while I was gone I placed plastic bags on top of the containers to retain moisture. The temperatures were probably in the mid to upper 80's while I was gone. Would the seeds have rotted, or was the temperature too high for germination? I know borcolli likes cool weather. I plan to plant some more now, and the temperatures will be in the mid 80's, but I do not plan to put plastic on them. I have grown brocolli seeds in the past with no problem.
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