mail in spam mailbox is invisible

Just switched from Entourage to Mail. Everything worked fine for the first week. My spam filter has been working well (VirusBarrier V4) and dutifully putting all spam into the spam mailbox. Starting this morning, however, even though the computer tells me there are, for example, 26 messages in the spam mailbox - none are visible. By clicking on one of the headers ("date received" for example) I can trigger the folder to display at least a few of the spam messages - but rarely all of them at once. ALSO, when Mail gets really nasty it ceases to display messages in ANY of the mailboxes. I see that other people are having trouble getting Mail to display messages. Maybe these problems are all connected. Sometimes the problem goes away (temporarily) when I open a new viewer window under the File menu. The only reason I'm using this funky program is that I bought an iPhone (which is awesome by the way!). Thanks for any help you can provide.