Two suggestions:
1. It could be too wet, How much are you watering it?
2. It could be becoming dormat. The leaves will go yellow and fall due to this.
The best way to find out what is happening with your tree is to see if you can find a club nearby and take it to there next meeting.
There is numerous clubs in South Florida, and you can find the one closest to you and find out there next meeting date by visiting:
http://www.bonsai-bsf.com/clubs/. Bonsai Societies of Florida has information about all their member clubs in Florida.
If you have any trouble finding a club near you, please email me and I will try and put you into contact me and I will try to help.
Also, look on the BSF website about there Convention in June 2008 with Cheng Cheng-Kung, Sean Smith and more very talented artist.
http://www.bonsai-bsf.com/events/2008/convention/ Conventions are great ways to learn.