Sounds like the neck may need to be adjusted. I've had that happen before and what's happening is that the strings are too close to the frets and the frets are deadening the notes, if that makes any sense. Try raising your action a bit by raising the saddle at the bridge. If that doesn't help, take it to a tech for a truss rod adjustment....

Most companies offer guitars with 24 frets, just have to look at different models. Ibanez and Jackson mostly have 24 frets.