I have only been playing a short time myself, about 2 months, but I know it can get a bit frustrating when your not getting the clear notes you want.
I would suggest a few things to try…
First, try blowing and drawing less vigorously. Sometimes when you blow too hard you can get a muffled sound.
Also, check the position of the harp in your mouth. Make sure it’s deep inside and you have the right angle. You might try adjusting the angle of the harp to see if the tone will improve.
Last, I might suggest making sure you change the shape of your bottom lip drawing the lip closer together. You may be letting too much air into the blow or draw so you end up hearing that muffled sound.
Being fairly new, these suggestions have oftened worked for me and I think they’re worth trying