Some pictures would greatly help us to diagnose more efficiently, but in the meantime, I can give a few helpful ideas.
Fukien Tea are very specific about liking lots of light. Supplemental light would be a great help - even if it were midsummer, I would still suggest this.
It is also very likely stressed from moving around... warehouse to store to house... This will cause leaf-drop while it adjusts to its new surroundings.
They also do not like to stay soggy. They prefer to mostly dry between waterings. Because they are usually imported from China, Fukien Tea trees very often come in that clay-based soil you described - which is highly water retentive and compacts quickly, creating drainage issues.
Also while waiting for pictures to further help, here are a few things to look at:
* Does it have any rocks glued onto the top of the soil? If so, these should be removed ASAP.
* Does it have drainage holes in the bottom of the pot? If not, this will also be part of your soggy soil problem.
* Does the water easily run out the bottom of the pot when you water the tree? (This is assuming their are drainage holes...) If the water pools on the top of the soil and does not drain quickly, this is a good sign that the clay your trees are potted in is compacting and not allowing water to flow through it properly. This could also mean that even though the soil is wet, the roots are not getting the water they need.
With some more information, I think we can get you moving in the right direction to make your trees more happy. Fukien Teas can be a temperamental species, though.
If you would know strength and patience, welcome the company of trees. ~ Hal Borland