Child support awards are always subject to modification. There are a multitude of reasons which would justify a modification, such as a loss in employment, change of custody, increase in income, etc.
The question which often arises is when does the modification, if granted, commence. The case of Webber v Webber, 56 So.3d 822 (Fla. 5th DCA 2011) makes it clear that the modification is effective, retroactive back to the date of filing the petition for modification, and not retroactive to the circumstances which give rise to the modification.
The point to note here is that if you are entitled to a modification, file your petition right away and don't delay!