Why is my lilac not growing flowers?

[ExistentialThreat]

It’s been in the ground about 4 years. It’s now looking healthy and about 4 foot tall, partial sun, but only has one flower bud. It was not pruned last year, not deadheaded. What am I doing wrong?

Try potash fertilizer for flowers.

My mum has one as part of a mixed hedge and since it’s been cut back less it’s flowered more. She used to be obsessed with having the hedge trimmed. If yours has been pruned only once that doesn’t sound like the problem. I’d go with feeding it. How many flowers did it have previously?

Nothing wrong, many shrubs take a few years to get going as they need to reach a certain level of maturity before they flower. Use tomato feed on it this year which is high in potassium (potash) and as a pp above says, this will encourage flowering next year.