Germination rates depend upon many factors, including the temperature of the soil or germination medium. Warmer soil temps will ensure a prompt response. Zinnias are warm weather plants and shouldn't be planted in situ until the soil has really warmed up.

Do you think that the soil temperature might be the limiting factor in this situation?

If not, some other possibilities could be when the seed was harvested and how it was stored. Sometimes (I have no idea that this is the case with your particular zinnias), hybrid cultivars produce empty seeds (with no embryos).

Personally, I don't save the seed from my favorite zinnia hybrids, as I know that they won't reproduce true but are likely to be quite a mixed bag of results. I don't have space to experiment (in terms of room for my germination flats), so I purchase new seeds every year. If I had the space, it would be fun to play!