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I could see two very possible options here. If the area remains well fertilized on schedule, then that can be eliminated.

The other option is a fungal infection. Although uncommon with onions... It can still happen. So it would really help to use wettable sulfur around the area if this is the case. Even if not, it will still help recondition your soil, especially if you use dolomitic lime with it.

Here is an article for growing onions: https://www.gardeningknowhow.com/edible/vegetables/onion/find-how-to-grow-onions-in-your-garden.htm

If possible, contact an extension agent through a university (with an agriculture/horticulture program) near you- extension agents are great resources since they understand your local conditions.

Here are some articles from universities in the US that might help:
http://extension.uga.edu/publications/detail.cfm?number=B1198-2
http://extension.psu.edu/business/ag-alternatives/horticulture/vegetables/onion-production
http://hendry.ifas.ufl.edu/HCHortNews_Onions.htm
http://vric.ucdavis.edu/veg_info_crop/onion.htm

Onions will, typically, bear flowers rather than bulblets. This is more characteristic of garlics. Most will do this. Are you looking for an ornamental variety or an edible?

There is a type of onion (arguably a garlic) that is called an Egyptian Walking Onion that would fit this description. This article will give you more information: https://www.gardeningknowhow.com/ornamental/flowers/walking-onion/growing-walking-onions.htm

They will, likely, flower this year. This means that they will not focus their energy on the "onion" part. I would suggest digging them up, and using the tops. The green part is often neglected, but you can use them as you would onions.

There is still a chance, as garlic will be more cold tolerant, therefor will usually show first. Depending on where you are located, it may not be warm enough for onions to come up yet. Especially if they are larger yellow onions, or species of sweet onions. They need warmer weather to be healthy.

It would not hurt to seed again. This can ensure that they will come up, whether they are from the previous planting, or from this one. Just thin them out if they are too close together, once they do come up.

This article will help you.
https://www.gardeningknowhow.com/edible/vegetables/onion/onion-plant-varieties.htm