A Eureka lemon tree
I only got one very small lemon on it and it stayed green. I have it in a pot.
There are many reasons for this to happen. If you do not fertilize it every few months then this can be the issue. I would also recommend having dolomitic lime, and iron sulfate on hand. You will want to apply these at least once per year.
The next possible issue will be not enough light. If this is indoors, then you will need supplemental light to produce fruit. If this is indoors, it would be wise to have at least 200 watts of horticultural grade lighting to produce fruit properly.
Temperature can also play a factor here. If it doesn't remain quite warm, then this can halt fruiting.