I am having problems barring all six strings on the first fret with my index finger. The string that doesn't sound is the D-string. The problem is: I don't know if my finger is placed correctly. I have tried to move it but no matter how I move it, some strings will not ring. The best position I have acheived is the one where my D-string doesn't sound only because on all other fret, I can barre all six.

The position I use on my index finger is with the upper joint of my finger on the A string but below that joint, any string there will not sound. Any tips? I have been practising for more tahn 3 weeks so can finger strength still be a problem?
I had this same problem when I was starting out. Finger strength is a part of the problem but it is mostly the position move your finger in different ways to get a ggod spot where they all sound cleanly. I`ll get easier with time.
Breakin` the Law!
Be sure your instrument is set up properly as well. An action that's too high can make barre-ing a literal pain.....