If you are sure about the identity of the tree, from a reliable source, then the next thing I would check is if the foliar crown is originating from the grafted scion/stem and not from below the graft. If a sucker has grown up from below the graft and dominated the foliar canopy, then it will not be true to the grafted variety. This is a common issue.
The next considerations are soil fertility and water management. Nutrient deficiencies and water deficit can cause physiological stress and suppress flowering.
You don't state if it's in the ground or in a pot, but in any case a good citrus fertilizer may help: