I would think it's the soil.
What you can do is test the seeds in a regular bag of organic soil. You can pick up one of these at your local garden store for pretty cheap.
if it still causes that problem then I'd think it's the seeds.
One question I have is, how much do you water it?
Under watering or over watering can cause all kinds of problems, and this could be something that is causing it to die faster.
Some food for thought
