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Are the signal meters showing full strength from where each device is? A signal can be strong in one area further away and weaker in another area that's closer to the source.
Just because a signal is strong doesn't mean it is clear. A strong signal with interference will be just as problematic. Interference can come from reflections off walls or metal objects, such as appliances, or it can come from other routers nearby using the same frequency channels. (Radio travels through walls, but they are only translucent, not fully transparent, so some of the signal is absorbed and some is reflected. The amount of reflection, absorption and transmission depends on the materials of the wall and the frequencies of the signal).
Channels can be changed in the router setup page. You can use a tool, such as an app on a phone to see the frequency channels and signal strengths of other nearby routers, and then use that to find a frequency that isn't being used nearby. Here's an example for Android that I use: https://play.google.com/store/apps/details?id=com.pierwiastek.wifidata&hl=en
If you have a mobile device with iOS, it needs to be jailbroken to run an app that can tell you the wifi signal strengths and frequencies around you. This is mentioned in this How to Geek article which also has tips for getting the best performance from your wifi: http://www.howtogeek.com/197268/how-to-find-the-best-wi-fi-channel-for-your-router-on-any-operating-system/
The article also explains how to do the same wifi analysis from Mac, PC and Linux machines. If you do change your router's wifi channel, you're likely to have the best performance by choosing from channels 1, 6 or 11. The in-between channels overlap some, reducing bandwidth and quality of signal. This is covered here: http://www.metageek.com/training/resources/why-channels-1-6-11.html (Also of note is that many routers these days can operate at higher frequencies. The old standard wifi frequency range is ~2.4 GHz, but there are many routers now that support ~5G Hz. This higher range of frequencies is less crowded, for now, and offers a few advantages. Some of these are covered here): http://www.extremetech.com/computing/179344-how-to-boost-your-wifi-speed-by-choosing-the-right-channel
The router in question might be in need of repositioning or being oriented in a different way. This is mentioned in this page, as linked to in the first on How to Geek: http://www.howtogeek.com/126327/how-to-get-a-better-wireless-signal-and-reduce-wireless-network-interference/
Another thing that can help is a parabolic reflector. If your router has an omnidirectional antenna (they look like a stick), then this is easily made from an aluminum pie plate and single-ply cardboard. I have used this method with success myself. Here's the example DIY reflector design that I used: http://www.freeantennas.com/projects/template2/ This is also discussed by PC Mag:http://www.pcmag.com/article2/0,2817,2418815,00.asp When I did this I was using a router with a single antenna in the corner of the apartment, so I didn't need the signal to be transmitted all directions, it only had to travel toward the rest of the suite. The reflector redirects the signal that would normally travel to the back and sides and sends it forward. This makes the total signal travelling forward much stronger.
If your router has several antennas, it may be difficult to use this type of reflector, and you will have to either make a smaller version, one for each antenna, or a larger version for all of them. The idea is to put the antenna at the focal point. If the router has a hidden, internal antenna, ideally you would open the router to see where this is, since it likely isn't in the middle of the case - but this may be impossible or void warranty, so instead, you could use a larger reflector and put the router at the centre. (Some metal bowls happen to be parabolic in shape, or close enough to help. For example, I had a bowl that had a polished mirror finish, so I was able to check to see if it was parabolic by holding it out in front of me and looking into the bowl. This allowed me to see my reflection uniformly on the surface of the whole bowl; if it hadn't been parabolic, my reflection would have only been on parts of the bowl at any one time). Whatever the reflector, if you use one, you can fine-tune the position by checking the signal strength as reported with any of the methods in the link above, such as with a phone app. (Some have had success using vegetable steamers; their fins with adjustable angles can provide desired reflection shape).
Finally, if your router has external, detachable antennas, it may be possible to get replacements that enhance the signal gain (higher sensitivity to incoming signals and greater broadcast strength). Failing all these, your router may be old or of a poor quality. I'd definitely recommend trying the above options though.