feeding leeks

Many plants will flower prematurely if stressful growing conditions make them "think" it's now or never to complete their life cycle.

Unusually large variations in temperature, lack of (or inconsistent) water, and lack of food are 3 possible causes of stress.

Leeks are of course a leafy veg which need plenty of nitrogen, and particularly in a container will need regular feeding and watering, although the slow release stuff should keep them going for a while.