onions

What type of onion, and from seed or from a 'set'? How far apart are they? Depending on the above, the chances are they will begin to bulb up a bit later in the season. Sounds like they are very healthy if they are making good growth. The main danger is of onions going up to seed without bulbing, so check no flower bud is forming at the top.
Onions like a rich soil so there should be no real danger they are being overfed. As long as there is a reasonable distance between plants they should bulb up ok.
(The only other possibility is that you are growing a non bulbing onion or the seed was mixed up by the supplier! But I'd bet you'll get bulbs later.)