Trust me on this one. You ARE sucking with your lips and mouth. It may not feel like it to you now, but you are. This is possibly the most common problem/complaint that I've run into in 30 years. Correct diaphragmatic breathing is the most difficult and complex technique of advanced harmonica playing. I've gotten close to 1000 emails and posts regarding this over the last 10 years online.
Don't worry about any of this, it will come when it comes. Most of my private students through the years continue to work on it after 2-3 years of practice, feedback, and instruction. It's tough !! Not every pro out there has perfect technique, but it doesn't stop them from making good music. For examples of how great breathing technique can make the harmonica sound amazing, check CDs by Paul Butterfield, James Cotton, and Sonny Boy Williamson II (to name a few).
Check out the bending section of the site in regards to bending and "not" bending. This is part of the problem also. Play around with tilting the harmonica up and down as explained on this page-
Get the harmonica as far into your mouth as you can (without losing the single note). This will make it difficult to suck with your lips even if you want to. See these posts on "Vertical Slot"-
Lastly, post an MP3 file of your sound on these holes online or email a 20 second clip to me to listen to. Info on setting up a free site-
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