I feel your pain!
I have a black sapote and a persimmon that have been in the ground since 2009, but have remained puny and stagnant ever since, in spite of all kinds of TLC from me. Their neighbors (lychee, mango, jujube, etc.) are all thriving and reaching for the sky, but these two are just pathetic. A few years ago, my $200 sapodilla suddenly died after being in the ground and quite healthy for two years.
I guess, like humans, some trees are just sickly and ill-fated, and there's not much you can do about it.

Thanks for your comforting answer Brute! Would it be safe to remove it from the ground and put it in a container with a richer soil? A foot below the surface the yard's soil is dune sand. Since sapodillas are slow growing trees, I want to plant a longan tree in that place if nothing changes when it starts warming up.