The tree looks to be drastically undernourished

I've been giving it citrus fertilizer once a month the same way I do my other citrus trees. Perhaps it needs something else?

Hmm. I just responded to another poster who has a citrus that also is failing to thrive. He posted that quite a lot of pine mulch, complete with wood chips, was mixed into the soil. I suggested that might be causing a nitrogen deficiency.

Anyway, I can see in your pictures that the tree is heavily mulched with wood chips. Normally, if wood chips are on top of the ground they should not cause a problem . . . but I can see an awful lot of them in those pictures, and your tree is quite petite!

Another possibility is that in applying the mulch you have smothered the tree's feeder roots. Someone with more familiarity with the cultural practices in your area should be able to help, but I would say that for such a small tree the mulch is probably not helping. In fact, it is probably hurting.

For that matter, your problem may well be a combination of both possibilities! Think of it this way: a mulch ring three feet wide and two or three inches deep is usually fine for a tree that is ten feet tall and about the same diameter, but you have a tree that is about a foot high and wide and a mulch diameter of what looks like at least two feet. Thus, for its size, your tree has a shallower and smaller network of surface feeder roots than the larger tree, but is located in a mulched area that is, relatively speaking, the equivalent of a thirty or forty foot wide mulch zone around the larger tree, with probably a similarly wacky depth to that mulch!

All of which is to say, I'd remove the mulch!

Thanks for the mulch tip. Unfortunately I doubt that is the problem as I just added the mulch within the last week or so to try to keep weeds from growing in the area. I'll remove the mulch and try to keep the area weeded and see if that helps.

It appears as though your tree is planted entirely too low. When you planted it, did you amend the soil?

Another thought (since the mulch idea was wrong!): you may want to cut back on the fertilizer. If the tree is not actively growing or flowering/fruiting, it is not likely to need a monthly dose of fertilizer.

It may be planted too low. It's planted at about the same level as it was in the pot when purchased but may have settled or been too low to begin with.

When planting this plant I used the soil in the pot and native soil - nothing else. The native soil is at least half decent as other stuff planted nearby seems to be doing fine.

I'll try cutting back on fertilizer and see if that does anything.

It does look like your tree is either planted too deeply and/or the mulch is piled too high next to the trunk. If you dig it up to raise it in the planting hole you'll also get a better look at the roots to see what they are doing. You might also try a foliar feed just to green up the leaves and strengthen it a bit until you solve the basic problem. I would use Miracid.