A mistake first timers often make is fully extending the index finger, laying it flat and straight on the barre position, then trying to place the other fingers. If you're doing that -- don't.
Hyper-extending your finger and fretting with its flat underside will create tension in your hand and leave it in an unnatural position where the other fingers cannot be placed without strain. What you want is a relaxed, slightly bowed first finger, with the fretting done somewhere between its underside and outer side.
Take a look at this guy's hand:
Note the curve in his finger. Note how he uses the space between frets to accomodate the curve, with the tip closer to the fret than the rest of the finger. You can also see that even though the neck is angled with with headstock closer to the camera, there is more of his finger's inner side showing. That's because it's slightly rolled upward, with the fretting done under the flesh toward its outer side. Finally, notice where his large knuckle is. It's not even with the fretboard, where it would be if he'd used a straightened finger, but back toward the body.
To get a feel for this, it might help to exaggerate a bit. Curl your index finger slightly and lay its outer edge against the fretboard. Then, bring the other fingers down into position, rotating your first finger just enough to facilitate their reach. Hopefully, you'll wind up with something like the photo. Now, squeeze a bit and strum.
If the bass notes are dead: check to be sure you're fretting behind the fret, not directly over or in front of it. If the fretting is good, apply a little more pressure by using the flex available in the joint at the index finger's tip.
If the higher notes are dead: again, check fretting. If the fretting is okay, DON'T squeeze harder. Instead, apply more pressure by turning your wrist upward or pulling your arm back toward the body. Or a bit of both. This will mash the lower part of your index finger against the fretboard without giving your hands squeeze cramps.