Corky bro the confusing thing here is what Barry and Ken are pointing out. A ladder braced back won't fit the rim in the head block area without a slight bit of encouragement and this is perfectly normal.
If what you are seeing is that the back when say sitting brace side down on your bench has a slight depression in the middle of the lower bout then this is indeed a RH problem and the braces need to come off and be redone under stable RH conditions with well acclimated bracing stock.
Many builders try to maintain something like 42-48% RH in their shops at all times and especially before and during bracing for exactly the reasons of what you may be seeing with your back. I have taken a braced top with a 25'ish dome built into it out of my shop onto my deck to sand (trying to keep up with Lance.....
![[headinwall]](./images/smilies/headbangwalluf8.gif "Mad")
) and watched it flatten out in 20 minutes time. For builders where maintaining proper RH is difficult it's a good idea to have the rim prepared to accept the top and back in every way possible except of course the pockets prior to bracing so that once the top and back are braced they can be attached in short order to the rim. Once the back and top are attached to the rim some RH fluctuations are not as potentially painful as when the plates are unattached.
If your shop has decent RH management with constant RH in an acceptable range braced plates can hang around forever awaiting attachment to the rim.
I'll take them off and go again (I think I'll wait a week - to let the bracing stock get used to the 50% humidity which will be the rule in my shop until I turn the heat back on in October.