File Sharing Issue with OS X 10.6.8 Server

I have a small office running 4 macs connected to a Mac Mini running OS X 10.6.8 Server. Some of the computers are connected via ethernet and some via Airport Base Station. We have a Drobo connect to the Mac Mini to serve files. I recently upgraded one of the connected computers (MBP Retina Mid 2012) to OS X 10.11.2. This computer is connected to the server via Airport. The issue I have on this machine is that folders and files are missing when logged into the server. A folder containing 4 subfolders will only show the first 3. I select one of these 3 and the folders within will be missing one folder. It is usually the last folder in the sequence, but not always. If I connect to the server with Screen Sharing on this same computer I am able to see all folders actually exist on the server. I'd like to upgrade the other computers in the office. Any suggestions how I can solve this issue?