I had that problem when I first started too. Make sure you're using the very tips of your fingers to press down the strings. Also when you get into the right position pluck all the strings individually to see if any if them are muted, and if one is try to look at your fingers and see which one of them is muting it and adjust accordingly.

And it also is just one of those things that takes time and patience. But just keep it up and after not too long you'll be switching between chords without even thinking about it.