Could be the seeds or variety or starting soil, etc. some grow better than others. Some take awhile to do anything!!! Apparently onion seed life is considered to be 1 year, so viability must drop a lot after a year.I attempted to grow onions from seeds last year and they did terribly. I grew the right seeds for my area (long day), started them indoors, planted them outside, and they just didn't grow.
This year I'm buying a bag of sets and a bundle of starts instead.
The sets or starts should work well for you. Put them in a very sunny spot!