>>>You can set controls in an updatepanel as triggers that will cause a full page postback when they change, or you can add any controls that will be affected by updatepanel controls to the updatepanel.
>>>
>>>As a test, you might put your updatepanel (and contenttemplate tags) around all the controls on the page instead of just a subset and see if everything updates correctly. If so, you have a good idea of the culprit.
>>
>>I'll try that.

I wrapped both groups of controls in one updatepanel and that worked! Thanks to you and Viv and anybody else who helped!

I would love to know why it works differently at the two sites, but time does not permit digging any more into this at the moment.