In my more than 20 years experience teaching, I've found difficulty with barre chords to be a symptom of seated position and use of the left arm. While it is very difficult to write about, it is far easier to demonstrate in person or via Skype. Many players eventually manage to obtain results through trial and error, and sheer determination. Moving to the wider classical neck compounds the problems.
In the studio, I often have my students do an introductory exercise. In a good default seated position, imagine the left hand holding an apple. Slowly bring the "apple" to the fingerboard. My experience has been then students have already made a mistake at this point, where gentle guidance and persistence has to untrain the fingers to "do something." Imagine the fingertips of the left hand outlining a straight line. Now imagine that line running on either a 3rd or 4th string. Using the shoulder, elbow, and wrist (like turning a door knob), spend time getting the fingers as close as possible to their targets (one finger per fret) without actually using the fingers. Repeat as often as necessary until it becomes easier for the shoulder fist, then the elbow, and finally the wrist to do most of the work. Then allow the fingers to do the minimal work. Place the 4th finger. Then the 3rd. And finally, the 2nd." Now, meditate on the 1st finger. Allow it to reach its destination the easiest, laziest way possible. This exercise cannot be practiced too much.
Once the exercise becomes more familiar, imagine the line of the fingers NOT being parallel with the strings. Imagine it being more diagonal, or even forming a right angle from the strings. How do shoulders, elbows, and wrists compensate then? Mediate on on extreme counterclockwise turns of the wrist bringing the elbow close to the body. Imagine extreme clockwise rotations bringing the elbow away from the body.
Finally, apply this knowledge to some actual barre chords. Always use combinations of shoulder, elbow, and wrist to place 4th, 3rd, and 2nd fingers in place. Then it is time to experiment with the 1st finger. Usually, a 1st finger is only required to play the first and sixth strings, or the first and fifth strings. Further, if everything above is done well, the 1st finger is usually naturally approaching the fretboard from its side: often very counter-intuitive for many people. By experimenting with subtle movements of the first finger, by either moving upwards or downwards, or by flexing or extending more; students begin to discover ways that require far less effort. The movements require far more accuracy with far less margin of error. As a result, the desired goal is usually not readily available. As a solution, most students use far more effort than is necessary in order to achieve more immediate results.
I DID warn you that this is difficult to write about.
If we can find a suitable time that works for both of us, I'd be happy to take a quick look on Skype sometime. Warmest regards, and happy practicing!
Dr. Todd Tipton, Noda Guitar Studio
Charlotte, NC, USA (available via Skype)