I wonder if it is getting enough sun? I have a lemon tree in full sun which is quite productive, but last year was a poor one in my garden for citrus. Have you tried posting on the citrus forum? There are many folks over there that probably could answer your question.

Frank 2007, have you checked the age of the tree? For instance, trees that are grown from seed can take up to 5 years or more to fruit. Others that were airlayered may or may not fruit the first few years (I have one lemon that was airlayered that has fruited, one that has not fruited; both are a year old and the fruited one is only about a foot tall). Some trees that are grafted will produce none to a few fruits the first year and can take up to 5 years for a good crop.
Regarding fertilizer: I have a very challenging soil which is alkaline. Citrus require an acidic soil. I can always tell when I need to put another application of citrus fertilizer when I notice that the new leaves are pale.
Hope this helps,
Lee