Hitting the wall
Walls and furniture can weaken the signal from range extenders and routers alike. While most modern internal walls aren’t too much of a hindrance, brick and extra-thick walls can reduce signal strength.
You can try to combat this by placing your range extender next to a window or even up near the ceiling where there are fewer obstructions.
Alternatively, you can use ethernet cables for part of your home network. These have little to no loss of throughput speeds. The only way to achieve close to ethernet cable speeds on a WiFi network is to use a dual-, tri- or quad-band router along with a dual-band range extender.