It's not that he couldn't tune it, period, it's just that there was so much going on during The Wall shows which Roger needed to focus on and direct, that he didn't have enough time to just stop and tune the bass between songs.

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what do you teach your children about me?
what do you teach your little children about me?
pimp, thug, bling drug lord of the undergrounded kings
how can you be so sure i won't call down the rain?
what do you teach your little children about me?
you point your gun, wait, hide and run. i see it plain