Two years ago I had many, many marigolds go to seed, so they dropped thousands of seeds at various places around my garden. However, to my knowledge, those seeds didn't produce any marigolds at all (and there are none that have come up this year either). Any ideas as to why that might be?

Garden is zone 4a (MN). Soil is fairly friable, full sun. Other plants seem to do fine with self-propagation, e.g., sunflowers, tomatoes, radishes. I think all the original marigold seeds were Burpee, a few different varieties.