Lately ive been playing my classical guitar and im finding it difficult to get all the notes in a barre chord to ring properly. I can get most of the strings but most of the time the B string doesnt ring. This makes songs like "Cavatina" difficult to play.

Are there any exercises or tips to help this or is it all down to practice practice practice?
Mostly practice practice practice. However, if you kind of slide your thumb to the side so that its not directly behind your index fingering it gives you a little more torque to strengthen that barre up.
If you're a classical player practice them on a steel string, get decent at them on that and playing on a classical will be a breeze
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yea exactly, i see practicing on steel acoustic will build up ur strength alot faster, classic will be easy for u then.. and yea jst keep practicing, try figure out which position suits u btr, while making sure theres no extra tension on ur hand, the force should only come from the fingers, and arms, not the wrist.
i have the same problem on acoustic
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Aside the "practice practice practice" theory. I use a 6 pound dumbbell for wrist curls. I brace my arm and do about 30 reps before playing and a few other times during the day. I find it helps a lot by stretching and strengthening the muscles on the back of ones hand. As well as hand exercises of rolling fingers into a fist and rolling wrist forward, makes playing feel more effortless. Same idea as warming up before going for a run/workout.
Try practicing barre chords half way up the neck, and then working your way down to the first fret. Should be easier to get a full barre that way.

I wouldn't think of it in terms of barre "strength", as it is not a matter of insufficient brute force pressing down the strings. Think "sensitivity", and whether your finger is pushing down on all the strings you need to barre. You don't need to strangle your neck to do a full barre.