Zigbee does not Mesh properly

Hello,

i´ve just set up an Raspberry Pi 4 with Home Assistant and everything works well so far.
I am using Zigbee2mqtt and the Sonoff 3.0 USB adapter as coordinator.

For now i have only added some thermostats, temperature sensor, door contacs and switchable power sockets. The permanently powered sockets and one switch are connected to the coordinator as router device and also building some mesh line on the Zigbee map.
The non-router devices are not building a mesh / don´t connect to two or more routers or coordinator.
They only are establishing one single connection and unfortunately not the best signal quality connection.

We have three floors, at the 1st floor in the living room is the coordinator. In the kitchen, next room to the living room, are three power sockets, which are connected witch each other and to the coordinator.
At the 2nd floor i have a 230v powered switch device from Sonoff, which is also meshing to the router in the kitchen and connects to the coordinator.

At the 3rd floor i have a thermostat, which does not connect to the nearest and strongest signal which would be provided by the switch in the 2nd floor, it is only connecting direct with one connection to the coordinator with a poor link quality. Due to this por connection the device isn´t reactiing to commands every time. Sometimes your are setting the temperature 10 times and nothing reacts.

Did i have made a mistake while setting everything up?

A friend of mine also having a Pi with HA and his thermostats, tempertaure sensors etc. are building more than one connection to improve link quality.

Have anyone an idea what was is going wrong with my setup?

Kind regards,
Michael

Try to add a second mains powered device on the 3rd floor. The more mains powered devices you have, the better your mesh will be.
And it seems normal that battery powered devices only create one link.