Email Help

Ok I did search the forum before I started this and didn't find my exact problem so here I go . . .

I have a MacBook running Snow Leopard and I have used the Mac Mail Client since day one. My email service is through my BellSouth DSL which is provided through Yahoo ... pop.att.yahoo.com ... just recently mac mail has started NOT downloading everything in my inbox. I will open the app and the little blue bar will pop up and it'll say downloading 1 of 5 or w/e and it disappears very quickly without d/l'ing any new mail. If I log in to the webmail version I will have multiple new msgs it is seemingly refusing to d/l ... I tried changing mail back to all of its default settings and this did not seem to help ...

Any suggestions please!?

-UPDATE-

I have several custom "rules" I made over a year ago in Mail. I disabled them and now I get all my incoming messages ... but I don't understand why these rules I've had for so long would cause an issue now ...

Still appreciative of any input on this issue