Most of the beginners tend to have problems with these closed position chords. Common barre chords are in E and A forms. They are often difficult to play because the index finger is set across the fret board and it has to push a little bit hard to sound all the strings. But in order to learn these easily, fingers must not be stiffed. The same rule applies to most of playing techniques in both guitar and piano.
Your index finger works the same way that the capo does. A capo is put on the neck of the guitar to raise its tune. For instance, when you put your index finger across the fretboard you are raising its tune by one fret if you are in the key of F. If you examine the E form diagram you will note that fingers 3,4 and 2 positions resemble that of the key of E. This is why it is called its name. This is also true to A form.
The other position of A form is widely used because it only uses fingers 1 and 3. This means that you can move smoothly on the fret everytime there is a chord transition. In this position, finger 3 will also be pushing string nos. 2,3 and 4. When you strum your guitar downstroke it is inevitable to hit string no.1 but it will be muted or dead because the same finger is slightly touching the said string. Furthermore, the common chord position for B is in A form while F is in E.