I am currently working through Blues Level I. While most of it has been going very well, I now find myself unable to play the thumb voicing of the C7 chord. I can bar the D, G, B and high E with my first finger without difficulty. However, when I try to add my third finger I find that none of the strings ring out at all.

I am currently playing a full sized acoustic guitar. Does anybody have any tips? Is this even possible on and acoustic guitar?

I am willing to keep trying as long as necessary as I have already learned that we all have specific challenges that we can work through with persistence.