Hm, I'm also having a problem with my Ulocked GSM Pre 2 and my yahoo email.
I noticed it yesterday, but I think the problem started the night before, so 6/22.
My problem sounds very similar to the OP's description. New mail had simply stopped coming down from my Yahoo account. It's my default account, and I have 2 other gmail accounts set up on my Pre 2.
Hitting the refresh button didn't help. No new mail came down.
I tried 1) removing the account from within the email app (I think...tried so many things, I'm not *entirely* sure I did this one), then re-adding it--didn't work, 2) removing the email part of the yahoo account in Accounts and re-enabling it, 3) I tried removing the account entirely and putting it back in. Didn't help--no mail came down.
What did seem to help was removing the account in Accounts, then rebooting it, *then* re-adding the account with all relevant apps enabled, including email. That *did* work, partially...
..so I did get mail to come down, and newer mail from after the previous stopping point. But only the from and subject lines showed in the inbox scene for that account. When I click into an email, it tells me that it has to download the content of the email.
But the last email that came down was at 8-something this morning, and I definitely have email that came in after that. And now it's back to the completely ineffectual refresh-but-nothing-new-comes down state.
There's no setting I can find in the email app's account settings to just download header info.
And just now, I disabled the email component of that account in Accounts (after which I got a banner notification that the phone would delete that info), but the email app still has both that account and all the incompletely downloaded email from that account.
I think I'll try rebooting now before I take further steps... 'bye for now (as I'm typing this on the phone in question).
UPDATE AFTER REBOOT: The email & account were still in the email ap. Then i went back into the accounts app, and that Yahoo account. The email part was still enabled! So that's when I relearned soomething about webOS settings screens--the guideline design behavior is not to have a save button, and for the back gesture to do the saving...and I hadn't back-gestured out of the that accounts settings screen before this last reboot. So this time I disabled the email part of that account, and backed out of the screen. Account is now gone from the email app.
Now...let's see what happens when I re-enable it...
...it downloads new email, but it's missing the most recent one, which the mobile Yahoo mail via the browser shows came in "now" (previous one is 15 min ago). However, the email app on the phone still is only downloading the header info for that account, whereas the one gmail account that I've tested does get the whole message down automatically, w the first line of the body visible in that account's inbox view.