Certified GKH Gardening Expert
Depending on whether you are talking about the Orange or the Plum, there could be a few reasons for the lack of fruit production.
If this is the Satsuma Citrus that you are referring to, then it is likely a soil fertility or pH issue. Here are some articles that will help:
If this is the plum, then it is likely that your area is too warm (or rather doesn't receive enough cold) to produce. It could, still, be a nutrient or fertilizer issue, but plums will enjoy a cold period during the winter. Here is an article for more information on why plums fail to fruit: