By 310310
United Kingdom 
i've a common lilac shrub planted in my garden for about 2yrs. we bought it just after it finished flowering, it is growing fantastic but unfortunately it hasn't flowered since. Please tell me why this is as its flowering everywhere else but mine. if i'm doinig anything wrong let know.Thanks
1 May, 2011
Welcome, 310310!
The bush may just be too young. Growing in a pot at the production nursery will cause it to mature early, but only until it is planted out and can "stretch its roots." Once planted, it will probably spend a few years putting on some size, before it blooms again. There are a few things that you can do to encourage it:
Make sure that it is getting at least 6 hours of direct sun a day. Too much shade can make bloom sparse even on old plants.
Feed with a fertilizer low in nitrogen, and high in phosphate and potash, such as tomato food. Abundant nitrogen will delay maturity.
Don't prune any more than you have to. Blooms appear on the tips of the shoots from last year, and if you try to hedge it, the bush may never bloom.
Hope this helps!
2 May, 2011