Folders / files disappearing, unable to see on another Mac RN31400 – ReadyNAS 300 Series 4- Bay

I've been using my ReadyNas for over two years now with no issues, within the last 6 months it has begun to make files "disappear" until the entire drive is rebooted (this involves saving and closing all documents that are connected to the drive, quitting all programmes, relaunching the finder window, ejecting the drive itself, disconnecting myself then re-entering my log-in details. Pretty tedious when you have to do this three times a day)

We have 5 people working off the ReadyNas, if someone creates a folder on their computer and asks me to access a file within that folder I follow the chain of folders to its location and can see the folder. Once I click on the folder is automatically disappears. This folder can be seen on others computers but not mine until the drive is ejected and rebooted. This happens to all five of us when someone creates a new folder / saves down a new file. 

Any suggestions, tips or thoughts would be hugely appreciated.

Model: ReadyNas 314

RN31400 – ReadyNAS 300 Series 4- Bay (Diskless)

Thanks, 

Melissa