Many contacts for same extension not stable

I am using Asterisk 13 with FreePBX 12, I made an extension 40 and I have 3 contacts on it. i am using zoiper as the client on a MAC, iPhone and and iPad. I have set it up on all 3 devices.

The iPhone and iPad seem to be registered all the time, but on my Mac, the extension un-registers in some time, I thought maybe I need to register it after giving the number of contacts and tried that. But after a definite period of time, the extension on my MAC keeps un-registering. Does anyone know what could be the problem ?