Not all television or AVR models can be controlled over a network. Generally speaking, if the brand is supported in the Volume plugin then it should work on late model televisions or AVRs of that brand.
One good way to determine if you should be able to control it or not is to try installing another app (official or 3rd party) that is supposed to control your device (see below for more info). If you can control your TV or AVR with another app, then you should be able to control it with the Volume plugin.
Another thing to consider is to make sure you have the correct IP address set up in the Volume plugin app. Most official or other apps have automatic discovery of your televsion or AVR. Because the Volume Plugin supports so many various device brands, it was decided (at this time at least) to limit it to manually entering the IP address of the device. So if it is not working then that is one of the first things you should check.
If you are still having problems and you are certain that your device should work with the app but it doesn't, then please send me an email with details on your device (brand, model, etc).
If the apps below can control your TV or AVR then the Volume plugin should as well. If the apps below cannot control your TV or AVR then you may be out of luck. This could happen on slightly older devices, models without network capabilities, or just varying model numbers.