Bob: Laburnums can be a little iffy to flower. It could be that it's a grafted named variety like L. x watereri 'Vossii', in which case it could possibly be the root stock coming up as well and not looking so happy. Check that the leaves look the same all over. It's also possible that it's a chance seedling, and not very floriferous. But most likely, the last time it flowered it produced an abundance of seed and exhausted itself, and that's caused it to take a couple of years off. If it's flowered well this year it's almost certainly going to set a big crop of seed again. It's the same with lilacs, and fruit trees of course, if they produce a massive crop of seed they put themselves out of flower for a year. So remove all the seeds that form. It's a good idea anyway because laburnum seeds are poisonous, and if a child ate them they could accidentally kill themselves.