Enable doesn't work? Says successful but isn't


I have a couple of plugins that are not flagged as active even though I enable them (and get a success message).   I can still add other plugins and enable them fine (even variants of the two that give activation confirmation but don't actually update).

Removing the errant plugins and adding them again/turning off simplecache etc doesn't help.  In the diagnostic for each I get   [active] =>   instead of   [active] => 1

Any suggestions?  Am thinking I need to flush something else or nudge something in the mysql backend, but I don't know what.  start.php etc are all fine, so its not code...