Most Wi-Fi problems are caused by interference. This is because the majority of networks operate on the 2.4GHz frequency band, which is also used by a host of other devices, including baby monitors, DECT phones, Bluetooth headsets and game controllers. That's not even counting all of the other wireless networks out there competing with your own.
Indoors, the construction of your home has an impact: wood, plaster and glass have little impact on 2.4GHz radio waves, but brick and concrete can impede them; in some cases, if your router's mounted on a brick wall or sat on a metal shelf, simply moving it can make a difference.
Newer routers can also work on the 5GHz band. This has the advantage that there's far fewer things to interfere with the wireless signal, with the frequency reserved for networking; the downside is that range of these networks is a lot less than with 2.4GHz networks.
Whether you've got a 2.4GHz or 5GHz network, the key to a trouble-free life is to minimise interference where possible and use additional kit to extend range where physical factors (range and the construction of your home) are the limiting factors.