i know it sounds dumb but just keep practicing and make sure you press hard
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Try doing it more towards like the 7th or 5th fret first, its easier to do higher up on the neck, then when you get good at that move to lower frets
the problem is when i do it, the high g and b strings are often not pressed hard enough because thats the part of my finger (this is going to sound funny) where the soft fat is.
Step 1 is to find a positionthat works for you. Try mivong your index finger up and down and rolling it back and foth until you can get all of the strings down. This is usually done by rolling the finger towards the nut a little bit and having the middle knuckle just below the high E string. In other words, mostly the top half of the finger will be used. Then, start practicing until you build up a caluse and can jump right to a particular chord.
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Try to roll your index finger over to its side a bit. It's easier to press down that way. Hope this helps.
I was having the same problem. I took my guitar to the shop, and the guy told me that my action was too high- get it checked out. It made the world of difference for me.