It could be a number of things. If it was the fault of the seed just not germinating ... it could have been a seed that just wasn't viable. I do the squeeze test. If you squeeze a seed between your fingers (AFTER it has dried) and it's smushy, that seed is no good. It should remain hard. Sometimes the seeds do germinate, but soil gnats larvae get to them and kill them. Fungus is also another cause of early seedling death ... like root rot. (Which is why I add a few drops of H2O2 in the water when watering all my seedlings and water only when soil is dry.) And some seeds might actually appear not viable because they may take several months (not weeks as most do) to germinate.
