Thank you for your question. The answer depends if you already have an order for support in your favor. If you don't then generally, you can only ask for support retroactive to 24 months prior to filing the petition for support. F.S. 61.30(17).
If you have a support order that provides for support, then if you can't find the parent, you may have to try to serve by newspaper publication. There is a procedure for doing this and you can find some forms that may be helpful at http://www.flcourts.org/gen_public/family/forms_rules/913b.pdf.
Feel free to follow-up and good luck.
AK
