I am a fairly inexperienced gardener but I do know that with some plants it can help to pollinate them by hand using cotton wool buds (q-tips). I had to do this last year with my chillis. Or perhaps it is sterile, where did you get the seed?

it mite be doing well but maybe its still not quite good enough for it to fruit.i think youve done realy well to grow one.

Have a look at what your feeding stuff is made up of. The main ingredients,(N.P.K.)eg 20.5.30. on the packet says it all.The first number refers to nitrogen percentage which encourages vegitation (leaves), the middle number refers to phosphorus which encourages the rooting system,and the third number refers to potash(potassium) which encourages flowering,thus fruiting.I think you are feeding too much nitrogen which means your tree is kept in the vegatative state,and the tree will not produce flowers at all.
As for growing a citrus tree from seed it is not recommended at all. A good chosen citrus variety should be bud -grafted on a bitter orange (seville orange) plant. From seed you will have a poor root system when it comes to produce fruit.But in your case,after having it for so long,I would not suggest starting over.sorry for the long answer. Regards.