Fukien tea scientific name is Ehretia microphylla or the synonym Carmona retusa, I don't know which name is the ''currently correct'' scientific name. The name will help you look up information about its growing habits.
Now to your question, I don't think fertilizer is your problem. I think the tree has been 3 years in the same soil since you got it, and maybe a year or more depending on when it was potted up by the seller. So 3 or 4 years without a change in soil. As various components of potting media age, they can break down. I see a little bark in there, and bark normally lasts only 2 or 3 years, then it degrades and becomes smaller and finer particles. If there was a calcined clay product in the mix, it could have broken down, going from discrete particles, like gravel to mud. This mud would plug up the air spaces in the potting mix. The collapse of the clay into mud can suffocate roots.
So my opinion is this tree needs to be repotted. The old potting mix needs to be thrown out, replaced with fresh mix. I assume you are in the USA, spring is just around the corner, go ahead, get your materials together then repot.
Fertilizer, trees can usually go a long time, years to decades in a pot or container, and be reasonably healthy without the addition of fertilizer. It is very, very unlikely that lack of fertilizer is your problem. I think you have an air penetrating to the roots problem.
While waiting to get ready to repot the tree, be careful to not over water, or under water. Collapsed soil, that has turned to mud, will stay wetter longer than potting media that still has good structure.
Fertilizer pelletized as slow release fertilizer. These fertilizers are useful in certain applications. Nurseries love them because it lowers their labor costs. The coatings used for the pellets have their release of fertilizer controlled by temperature, not by the presence of water. The hotter it is, the quicker the fertilizer will leach out of the pellets. Normally this is good. But when it gets really hot, over 95 F, the tree goes dormant, and does not need fertilizer. This is exactly when the pellets will dump their load at the fastest rate. Also during cooler weather they do not release much fertilizer at all. Your tree may be growing, but not getting the benefit of fertilizer until the weather gets above 60 F. For some bonsai trees, we control when fertilizer is applied. For example white pines we only fertilize after the foliage has extended and hardened off. So for the Chicago area, Sept, and October are the only 2 months I apply fertilizer to my white pines. If you mix the pellets into the potting media, you can't take away the fertilizer. If you put the pellets in a mesh bag, set the bag on the surface of the potting mix. THen you can put on and take off the fertilizer as needed.
So pelletized fertilizer is useful in some settings but is a mixed benefit, problem for bonsai culture.
