Device Agents Offline?

Hi all, I run an instance which has been slowly growing, we've recently hit 549 devices.  However, the amount online has stayed consistently just under 40%. 60% of devices will be offline at any time. IT also seems to rotate through different devices but never going above that 40% online mark.  All devices are Android +. Some will connect one day and not the next. I'm at a loss as to why this is happening, but we urgently need as many of these devices connected at once as possible. Would there be a connection limit on the server? Any help as to why this is and how to fix it would be hugely appreciated.