Hi there,
knowing nothing about your firmware does not help provide useful comments.
However, there could be so many things that affect the stability of a device:
- the wifi connection or the router stability
- if there is a microwave between them and the router (they generally use 2.4GHz as well)
- a power outage
- a new feature in the firmware
- if best coding practices are being used
- etc
I have few devices at home, and every other power outage or something (whatever, a solar storm? just kidding or maybe not so much 
) I have to reset one solid device that's been running for years. Maybe this happens once or twice a year per device.
If I multiply this by a factor of 10 or 100 to match the number of customers you have, it could simply be a similar situation.
One way out of this, and in order to add robustness to your product would be to add a watchdog.
You can read more here and there (see below).
I would recommend you add one to your product, and there would be no more need for a customer to reset their device (ok, maybe you'll be covered for up to 99% of the cases, let's be realistic and conservative here).
Here's a story about other fields using watchdogs:
Launched in January of 1994, the Clementine spacecraft spent two very successful months mapping the moon before leaving lunar orbit to head towards near-Earth asteroid Geographos.
A dual-processor Honeywell 1750 system handled telemetry and various spacecraft functions. Though the 1750 could control Clementine's thrusters, it did so only in emergency situations; all routine thruster operations were under ground control.
On May 7, 1994, the 1750 experienced a floating point exception. This wasn't unusual; some 3000 prior exceptions had been detected and handled properly. But immediately after the May 7 event downlinked data started varying wildly and nonsensically. Then the data froze. Controllers spent 20 minutes trying to bring the system back to life by sending software resets to the 1750; all were ignored. A hardware reset command finally brought Clementine back on-line.
Alive, yes, even communicating with the ground, but with virtually no fuel left.
Moral of the story: a watchdog can help you succeed in your mission.
source:
Best!
Gustavo.