Certified GKH Gardening Expert
With container grown citrus, the most common reason for lack of flowering will be a combination of a few factors.
Lack of lighting, improper watering, and improper N-P-K ratio will be the big trifecta, here. Typically it is a combination of all three, and this is especially true when they spend some of their time, indoors.
If you are not using a fertilizer that it made for citrus, then you can opt to blend your own. Just be sure to cut back on the nitrogen during the time it needs to flower. The focus should be on potassium and phosphorus, as well as the micro and trace nutrients. Though, citrus fertilizers make it much easier, since they are already blended for them.
Lighting should be bright. Window lighting will not be enough. I find that a fixture of, at least, 200 watts of horticultural grade lighting will suffice. The more the better (and the more you can fit under your light).