If the screws always comm loose, you should replace them with new ones. Go to get new screws in the same size and a tiny screwdriver from a dollar shop or other little stores. Then insert the new screws and tighten them with the tiny screwdriver by yourself.
If the screws keep loosing , it is better to replace them. Use a sheet of paper to cover under your work area ,and it can make you find the dropped screws easily . Remove the old screw with a thin-blade and insert a new one in the same size .Then tighten the screw until the frame is secure,but don't over tighten in case you could strip the screw .It is much better if you have an eyeglass repair kit which contains a tiny screwdriver designed to fit eyeglass screws, several screws in different sizes, a small magnifier and other replacement parts.
There are some small stores selling repair kits which contain a tiny screw driver and different size of screws. You can replace the loose screw with the tools by yourself. Or if you don't find these stores, just go to find a optical store, they will fix it for you.