I find that in learning any new chord and switching to it is best done slowly and in a round robin format.
take the chords E, A, F# (or F# minor). Start with B and make 4 slow strums, go to E and make 4 slow strums, then back to B for 4 strums, then to A for a measure and then back to B for a measure then to F# and to B. Take it slow and make sure your B chords and all the other chords ring true. Use chords you can play and add only 1 new chord at once. The reason for this is simple. It allows you to concentrate on that ONE new chord. This is important.... THINK AHEAD... think about where you're going and what your fingers have to do to get there. Are there any fingers that do not have to move, or stay in the same fret moving up or down a string.. or are on the right string but in the wrong fret and can slide. Use those fingers as anchors. In the beginning it is natural to move one finger at a time, as you get comfortable and the fingers know where they are going, they will move faster and all of them will seem to move in unison. It takes time.
If you're speaking about the B chord which is an A Form Barre chord at with the second fret Root, try this. Play an A chord with your 3rd finger placing your first finger behind the nut. Then slide your left hand down 2 frets.
If you're speaking about the B7 chord played like this
e 2
B 0
G 2
D 1
A 2
E x
make sure that your elbow is tucked into your body. I know I have a problem with making this chord ring when my elbow flies out like a chicken wing. This turns my fingers 45 degrees and sometimes mutes the D# and the open B string.
Hope some of these ideas help.