There are two ways you can have email messages shunted off to OmniFocus:
1) start the subject line with "--"
2) send the email to user+omnifocus
You have to configure it one way or the other.
That said, there are a couple of obvious points of failure.
1) you don't get the message
2) you get the message, but it can't be given to OmniFocus
Some mail servers don't allow you to do the user+omnifocus thing. You'll be able to recognize this with a simple test; send a message to user+anything and see if you get it in your mailbox. If you don't, you'll have to use the "--" subject line prefix to use this feature.
If you do get your test message, the next thing to check is whether or not your email has a body in rich text. That doesn't play well with the Applescript that runs to transfer your message to OmniFocus and create the new action(s). Plain text only seems to be the name of the game here.
If neither of those are the culprit, then I'd make sure you don't have some other rule that is intercepting the message from being processed.