Wi-Fi is a mysterio, there is not a single dart that can be thrown and say here is your problem. Signal: How many walls does it need to transmit through. What materials absorb/kill the signal. Do you have reflective or dissipative surfaces that kill the signal? Router How many devices are on the network? How far does the router typically transmit? do all of your devices play well with the signal transmission? Where is the router positioned? Have you considered Ethernet for non mobile devices? Carrier Are you overtaxing the current band with? . there is a whole lot of questions