A GFCI tripping can sometimes be difficult to pin down. Hopefully it is due to an overload, and CHW listed how to troubleshoot that. If it is a ground lead, that is mainly caused by moisture or a bare or loose wire coming into contact with something it is not suppose to.
In addition to the good information form CHW, I recently learned that they do wear out. I recently had to replace one. I've seen life spans listed to replace every 10 - 15 years, to replacement 15 - 25 years. The one I had to replace was 17 years old. I have another GFCI that is about 18 years old and still working fine. Signs that the GFCI is weaning out, is that it trips frequently without any other reason, or the reset button will not stay set.