There are two ideas that can usually help a guitarist who has trouble
with bar chords. One, it is not how strong you can squeeze, it is how
effectively you use gravity. Two, it is not how hard you squeese, it is
where you squeeze Let me expand.
Try playing the following bar chord with out using the thumb on your Left
Hand. (if you are saying "huh ?" let me elaborate, if you don't use the
thumb you will have to use your arm weight and strength.
When you are able to do this you have tapped in to the secret of barring
well. Don't just use your hand strength, use your arms weight and let
gravity do some of the work. Combine this force with your hand strength
and you are on your way.
N.B. I don't mean you should always play bar chords this way, it is
simply a way to tap some unused force.