Be sure that your barre finger is far enough back from the fret that it doesn't mute any strings.

Try slightly different finger positions. Some parts of your finger work better than others. For example, the part right at your knuckle may not have enough flesh on it to apply the same pressure to a string as the rest of your finger.

Keep at it. Eventually, it just seems to work, and you'll wonder why you thought it was so hard!