Water. The biggest issue will always be water and moisture retention.
Second biggest is onions. I haven’t successfully grown nice large onions.
Discussion
1. Where are you geographically.?
2. What varieties have you tried?
3. What have you planted, starts, plants or seeds?
Starts have never developed for me, bundles of seedlings do very well though. By starts I mean basically the bags of baby onions they sell. They always grow great but never get size here, then they bolt.
The key to onions is getting them in the ground as early as you can and making sure the soil is nutrient rich. The reason is that onions are day-length sensitive. They start bulbing once the days reach the number of hours of daylight that the variety has been bred for. So, the bigger you can get the plant (each green leaf will be a ring in the eventual bulb) before that starts, the bigger your onions can potentially be. Fortunately, onions are pretty cold tolerant plants, so putting them in the ground a month before the rest of the garden is possible in many climates, especially if you use some simple protection measures like a cold frame or just covering them with a sheet at night.
There's a breakdown of the three basic day-length types on this page: