I have never been able to grow Brussel Sprouts well. They grow fine, start growing buds, but the top third of the sprouts get over sized, the bottom third do not get a chance to develop and I only end up with a few good ones.
I have topped them off around September but it did not help.
From what you are describing it sounds as if you may have a boron deficiency in your soil. Brussel sprouts are particularly sensitive to a lack of boron and you end up with small sprouts, not very many of them, and hollow stems.
In order to rectify this you need to add this to your vegetable garden. Luckily this is easy to do, all you need is a box of borax.
Add 1 level tablespoon of borax to 5 quarts of water and spray this over a 50 square foot area. Don't over-treat your soil for boron deficiency as it is very difficult to remove from the soil, once it is there.
In addition to the boron problem, you will need to make sure that you are planting the sprouts correctly, under the right conditions.
Brussel sprouts need a lot of fertile soil to grow in, as well as high concentrations of nitrogen and some magnesium. Nitrogen can be given to them by adding well-rotten chicken manure to the soil, and several handfuls of Epsom salts per square yard.
When you start planting either the seeds or the seedlings, you need to make sure that your soil has lots of organic matter in it.
The soil should be at a pH of 6.5 or higher. Add lime to the soil to adjust accordingly.
Secondly, Brussel Sprouts are a fall vegetable that normally takes about 3 months to grow to maturity, and the sprouts can be harvested just after the first frosts. It they are exposed to warmer weather, they don't do well at all.
Brussels sprouts grow better when daily temperatures average about 65 degrees F. or less. Anything over that, and you end up with loose, blowsy buttons, instead of the tight buttons that you are looking for.
Thirdly, the number of sprouts corresponds to the number of leaves your Brussel sprouts. To get good leaf growth you need to plant your Brussel sprouts out in late winter, early spring, making sure that the soil is well drained but moist at all times, heavily treated with organic matter, and planted out in full sun.
Always make sure that your plants are firmly planted into the ground when transplanting, and even as they grow, make sure that the soil is firm around the stems. Brussel sprouts hate wind, and wind will cause the sprouts not to form properly.
2 - 4 weeks after planting, once your plants reach 12 inches in height you will need to give your plants a side dressing of bone meal. Bone meal should be applied again when the sprouts are 6 weeks old.
Make sure that you don't grow Brussel sprouts in the same area as you have planted other brassicas in the past.
Hope this all helps to give you a better Brussel sprout season next year!