Pictures? Age? Rootstock? Fertilizer regimen? Type of soil? If you want a good answer, you have to give us a little help

Also, is it still inside or outside? What are the temps and the weather like? Are other citrus blooming? Grapefruit is general is somewhat late bloomer compared to oranges, for example. The latest blooming citrus I have in my garden is kumquat.

I bought it last year, then potted it soon afterwards, maybe early June or so. It was in a 15 gallon container. I didn't feed it at first, but fed it early January, and again last week. I'm using an organic fertilizer by ecoscraps, but just bought a non-organic fertilizer by Arizona's Best. It's about 5-6 feet tall, had fruit on it when I bought it. I think it's on a sour orange rootstock. A label from the nursery says "oroblanco on sour". I don't know what type of soil it's in. I told my yard guy to buy the best soil he could because it was being used for other things too.

The grate is on there because my cat started digging in it when I brought it in for the winter, and I had to stop her. It's still inside, but I'll put it on the patio soon. I'm having some trouble with pictures, but I'll keep trying. I kept it on the patio over the summer.

last year, I had a little trouble with spider mites. I sprayed it with neem oil, the sprayed it with water before bringing it inside. It dropped a few leaves over the winter. It didn't grow much if at all, over the winter. In fact, I don't think it grew at all since I bought it.

I don't have any other citrus. It has been kind of cold here, but the summer is very hot. My asian pear tree hasn't bloomed yet, and it's in the ground.

My guess is that you should wait for it to bloom when temps are consistently warmer. Blooming comes usually after the first signs of new growth.

It has been pretty cold. Last year was a lot hotter. In fact, the weather on the news said that it was about 20 degrees hotter last year on the same date. I'm glad it's cooler, but am looking forward to warmer weather. My flowering trees haven't bloomed yet. I'll try uploading the pics again.

More pics...

I can't give it more light as long as it's inside. I guess I could put another grow light on it. I'll feed it more often. Will that make a difference?

Yes

Not much point in fertilising if there's no growth and not much photosynthesis going on...