Server unreachable on some devices

Hello everyone. Latest version of HA OS Pi3B. Everything ok, but after about one hour, if i try to open web interface with a new device, i got “not reachable error”. So i tried to ping from my pc, and it also says “host not reachable”. Weird thing is that everything still work fine on the previously connected device. E.g. : I open HA from my phone browser, it works ok. If later i want to open HA from my pc, browser, i got error, BUT my phone is still able to connect. It is not a pc problem, because i noticed this behaviour also on other devices, it is like after some time HA closes new connections. Anyone knows if there is some sort of blocking to disable? Thank you in advance.