Hey Bob,

I would theorize that you are having more trouble muting open chord shapes because they are inherently harder to mute and move than barre chords. When you're muting barre chords you might just be moving the E and A shapes for example for your I (one), IV (four), and V (five) chords. In the key of G, these would be G, C, and D respectively.

Using the E shape for the G barre chord on the third fret is easier than muting the open G chord. Using the A shape for C (on the third fret) and D (on the fifth fret) is just inherently easier for muting and moving. Practice moving between C and D on the fifth string first with 2 measures each of eighth note strumming (one and two and three and four and - or down up down up down up down up). Count it while you play it. Experiment muting it and not muting it. If you record yourself you can hear a big difference in this. Once you start getting the muting down you get more control over the sound ringing out.

Practice G C G D (first, two measures each)
Then practice G C G D (one measure each)

The sound ringing out too long is a big problem, especially on bass guitar when you have powerful, low bass notes ringing out that you don't want, so muting can give you better control over that by stopping them from ringing out very long anyway.

Hm...I hope this helps. I realize this would be easier to convey if I were a teacher on Guitartricks.com and had videos. Perhaps I will post some videos on my own site soon.

Happy New Year!

~ Zac Sullivan, M.A. ~
http://www.fretzeroguitar.com

Music and a healthy life really go hand-in-hand:
http://www.lifestylebreak.com/family/music-and-life-a-partnership-for-health/