Zigbee bad vertical connections?

I have quite some zigbee switches, that act as repeaters. So I think, since my network spans from the front of my house, to the back, into the yard, inside my garage back there, to the gate at the back of the property, that I have a decent network.

But… I can’t seem to get it upstairs. I have my hub in the hallway, 3 meters away is a Sonoff SN26 that’s a repeater, it’s right nexto the stairs. Up the stairs, so maybe 3-4 meters diagonal up, there’s another Sonoff SN26 that I can’t get connected. Now I moved that one to the outside wall, where, within 5-6 meters are 4 other repeater devices, inside and outside the house.

I’m lost at how to get the signal upstairs…