The problem is that you used last years seed. Allium seeds are notorious for their short lifespan, you should use fresh seed every year or all your other efforts will be in vain. Max lifespan in ideal storage conditions is about a year for onions and leek. One option is to take your leftover fresh seed and dry them down to about 1-2% moisture using a food dehydrator or a dessicant and then put them in a sealed airtight container and freeze them. Done properly this can prolong their viability to 5 years+. When removing the seed from the freezer do not open the container until it reaches ambient temperature, this will prevent condensation from forming on the seed and possibly damaging them in their fragile frozen state.

-Ken

Thanks, Ken!