Why won't my Brooms spread?

My Brooms won't seed themselves!

I was pleased; when my early year experiments with Broom, both in the garden and in the countryside; seemed to be going very well. From the first year after being planted; they flowered, podded and grew incredibly fast.

Most of the ones I have planted up here are from Burrator reservoir, near Yelverton and also from some damp woods, near Okehampton. Following my apparent success from earlier plantings; I planted them fairly excessively in year 6 and 7.
In the wild; they seem to occupy the areas around bogs and rivers and need a certain amount of companion plants nearby, as grass alone dose not provide enough nutrients for them.

Reason for Concern

Normally, I wouldn't be too bothered if a shrub or tree hadn't propagated it's self after the first few years, but Broom is a very short lived plant. I would say 6 years would be about the expected life span, for one of these bushes. Unlike their cousin the Gorse; Broom rush up and give it all they've got, then they suddenly die, when they have reached about 5'/1.5m.

Usually, in the wild; many young plants set seed, &/or fallen over specimens can layer and root into the ground and therefore spread vegetativly, but not here. The Brooms look so good at the moment, but if I can't work out how to get them to seed themselves; they could literally disappear, so soon after arriving. I mean; look at all these fat seed pods below. It's like this every year, but never do any seem to survive, to make little bushes.

I hope to find a solution to this problem and would appreciate any feed-back on the subject!