Ok, good to know its not an issue with the guitar. Its quite normal to struggle a bit with chords, especially barre chords when learning them for the first time. What is most likely happening is that your finger barring the B string is lifting ever so slightly as you adjust for the positioning of your other fingers. It may also be that you're slightly changing your arm position as you move up the fretboard. It really doesn't take much change to go from a clean note to having notes start buzzing. I've played guitar for years, and still when I learn a new chord there is a definite adjustment phase I have to go through.

Good news is the cure is pretty straight forward. First, accept the fact that the note will play cleanly if fretted properly, because you tested that.

Now, just bar the entire 8th fret with your index finger, until you get all the notes to sound cleanly. Once you have that down, add the other fingers to form the chord, but one finger at a time, and as you add each finger, play each string to make sure they are all still playing cleanly. You'll probably get to a point where they don't, and then you'll know which finger is causing the problem. Just keep breaking down the mechanics of it until you isolate where you are going wrong, and then fix that problem.

Just like Christopher teaches, don't try to make music with the chord shape until you can play it perfectly. Get the mechanics down first, then make music.

Barre chords can be touch to play, and for some definitely a little more challenging on the type of guitar you have... I think its a pretty flat fretboard.
But don't get me wrong, doesn't mean you need a new guitar, just means you have to be a little more careful. Once you get though, it does become muscle memory and you won't have to think about it nearly as much to get the chord playing cleanly.

Good Luck and let us know how it works out!