I ran into this myself with a contact sensor on my back gate, about 40ft and two walls from the coordinator. It would work fine for a day or two, then just drop off and need re-pairing. Here's what I learned digging into it.

The mesh isn't as smart as people assume

Zigbee mesh networking sounds great on paper: routers repeat the signal, end devices hop through them to reach the coordinator. But it only works if you actually have routers in the right spots. A lot of people don't realize:

Battery-powered devices (sensors, most buttons) are almost always "end devices," not routers. They don't repeat signal for anyone else.
Only mains-powered devices (smart plugs, some bulbs, in-wall switches) act as routers.
If your edge-of-house devices are all battery sensors with no mains-powered Zigbee device nearby, there's nothing to relay them. They're relying on a direct, weak hop straight back to the coordinator.

What actually fixed it for me

I added a Zigbee smart plug about halfway between the coordinator and the back gate, specifically as a repeater. Within a day the contact sensor's LQI/RSSI reading jumped noticeably and it hasn't dropped since.

A few other things worth checking if you're in this situation:

Route table health: some coordinators/hubs let you see routing paths in their diagnostics. If a device is hopping through 3+ routers, that's a sign your mesh has gaps.
Channel interference: Zigbee channels 15, 20, 25 overlap with 2.4GHz Wi-Fi. If your router's on a busy channel, that can look like a range problem when it's actually noise.
Firmware: some manufacturers have quietly improved mesh healing behavior in updates.

The takeaway

Zigbee range problems are usually a topology problem, not a raw distance problem. A $15 to $20 mains-powered plug positioned as a repeater will usually do more for your edge devices than moving the coordinator around.

Curious if others have run into this. Did adding routers fix it for you, or was it something else (channel, firmware, specific brand)?