Unusually shaped floor plan, especially the back corner room in L-shaped building often suffers from the dead signal zone. You have to be in a very limited area of service to recieve WiFi signal proerly.

Physical obstructions makes losing usually the main reason to spotty WiFi signals. Wall, door, floor made with metal, brick or concrete come between you and your router could stop the signal spreading.

Some devices, for instance, baby monitor, microwave, wireless phone are all opearating on the same frequency as standard WiFi, which is 2.4GHz. This will definitely result in traffic jam of frequency when you are using single-band router.

Your neighbors' WiFi network may also degrade your WiFi performance by taking the same channel as your WiFi.

Every WiFi router can broadcast signal a certain distance. The farther your device from router, the weaker the signal, the slower the speed.

Some specific device hogs all of the avaliable bandwidth makes the other devices connected but no signal.