jquery_ui on multisite - won't install

i have a multisite installation configured thus:

mydomain.com/
mydomain.com/sites/all/modules
mydomain.com/sites/domain2.com/
etc

(i'm using Dreamhost for web. to make multisite work, "domain2.com" etc mirror the base install; when i point to "domain2.com" the first time, with an unedited settings.php file, install goes perfectly. these domains then use the modules at /sites/all/modules)

i have install jquery_ui module as per directions, including the jquery_ui files at
mydomain.com/sites/all/modules/jquery_ui/jquery.ui/ui

for the base install, no problems.
for the mirrored/multisites - nothing. i get the error "The jQuery UI plugin is missing. Download and extract it to your jquery_ui module directory. (Currently using jQuery UI Not found)"

i went to my datebase (phpmyadmin) & changed the status of the jquery module to "1" in the system table; it worked. it allowed me to install Date Popup, apparently with no errors.
on a 2nd multisite, i discovered that it was also necessary to set field "schema_version" to "0" - it had been set to "-1"

and here's what's really weird: this used to work fine. i have several domains in this structure that have had the jquery module installed & running just fine for at least a year. at some point, the ability to install the module has been broken. the ones that worked were D6/pre-19, the current. whatever was in use a year or more ago. all i know is that the 6.19, i cannot get this install without jiggering the database directly.

thanks