Hi , I have problems with my bar - chords

I can easily take the shape of a bar chord , but my g string is always muted , and if its not that one , its my b string , always on of those 2...

Okey , if I then push really hard on my fretboard , I get a huge and immense cramp in my thumb area , I also get this when I play to lang bar chords and powerchords.

How can I fix this ?
Could it be with another guitar with another shape of neck I don't have this problem ?

Or what can I do to play all the strings cleany without pushing so hard on the frets without having that annoying cramp ( trust me , it hurts like hell)

Well first of, practice. You should be able to do bar chords cleanly without applying much pressure. Try and take the bar chord without using your thumb at all. To get the G and B string to ring you need to move your bar finger around untill you find your sweet spot. It's all technique.
Twist your finger around about 45 degrees towards the headstock. You want your finger to do the barré with bones, not that much flesh. Also, remember that in chords like this: 355433 you don't need to barré the A,D and G strings, so you can sort of arc your finger. Getting cramps in your thumb is simply a matter of bad technique, so all you've got to do is experiment with twisting your index finger around a bit. Also, experiment with placing your thumb at different places on the neck, that might help a bit as well.
I'm a communist. Really.
I agree with Raziel. Also try letting the tip of your index finger peep over the top of the neck. You'll notice that you'll probably play the notes more cleanly.
If your hands cramp up, chances are your guitar neck is bigger than it should be. When you play barre chords, you can also "thumb" a bottom note to relieve the stress. Also, you may adjust the action of the strings. Action means the height of the strings above the fretboard.