MacMini gets lots and not showing on network

I have two macs on my network: MacBook Pro (Yosemite 10.10.2) and MacMini (Yosemite 10.10.2 + Server).

They see each other on the network, but after some time Mac Mini gets lost from network (internet still works) until I check/uncheck share files in the system settings - the next second it shows up again. It happens everyday. Very strange and annoying.