It is possible that your lemon tree is lacking in phospherous which helps to produce fruit.  Is the plant growing in a pot or is it in the ground and in what part of the country are you growing it?  You might try giving it a fertilizer that has plenty of phospherous in it.

First did they bloom? Should have bloomed last month unless they are in full shade.  Next citrus sometimes send up shoots from below the graft these suckers may look a lot like your grafted leaves but often do not bloom as they are rootstock varieties.