I am pretty new to bonsai, and my Chinese Elm is doing great. Is yours getting enough sunlight? During the winter, mine is inside and under a grow lamp, and now in summer, it is outside in full sun. My Chinese Elm grows continually, although it is best out in the sun. If it’s not getting enough light, my suggestion is to move it to dappled outdoor sunlight for this summer. Grow lights for this winter, then full sun outside next summer.
It looks like it could benefit from a good pruning/shaping, which can be done at any time. I would recommend trimming off all of that new growth at the top of the tree. You have a good looking trunk there! work with it, and be sure to shape the tree in a bonsai style of your choosing. Mine is getting to the point that I am trimming branches back as soon as there are 6-8 leaves, I trim back to there are 2 leaves left on the branch. Once the branches are growing where I want them with tight ramifications and small leaves, it will only need light trimming after that.
I love the pot, btw! what is the soil the tree planted in, and where did you get it? It may need to be repotted into a better soil mix and possible root trim if pot bound.
Google Chinese Elm Bonsai care, and you will find lots of info on it.
Good luck with it, it will easily become a beautiful tree if pruned and trained correctly!