If you recently received your Froyo OTA, you may want to check to see if you're affected by a nasty little bug in the Gmail application by which any secondary accounts don't sync automatically. Users of the Nexus One, HTC Desire, Droid Incredible, Sprint Evo 4G, and Droid 2 seem to be equally affected, so this is likely an issue across the board and we'll see more reports as more devices get upgraded. What's most frustrating is that while this bug affects Android, it's not part of Android itself, so the community can't take a look and find a quick and dirty hack while waiting for a more permanent fix from Google -- the Gmail apk is closed-source and Google property.
There are myriad fixes floating around, but they seem to be temporary at best. The only true fix at this time is to use a third party mail client, like K-9 mail to sync the secondary GMail account via IMAP. The bug's been filed, be sure to chime in at the source link and let the Googlers know if you're seeing this one. In the meantime, links to install K-9 are after the break. [Google Mobile support] Thanks Artur!