Harmonica Curses #1 - Everything is Hidden
The majority of technique performed on the harmonica is hidden. You can't see what I'm doing and I can't see what you're doing. YOU can't see what you're doing.
The tongue is responsible for all good and evil on the harmonica.
If your tone is thin... a note bends, squeals or stalls unintentionally... a bend goes too far or not far enough... your tongue is in the wrong place. If your tone is good and you're able to control a bend at the correct pitch... your tongue is in the right place.
Get to know your tongue.
Close your eyes and move your tongue around, identifying where the back, middle and front of your tongue is. Identify where the different parts of your mouth are. Feel with the tip... then the middle... then the back. Try some bends and tongue block techniques... start to identify where the tongue is when you sound good and where it is when you sound bad. Use speech to identify locations (sss, sh, ch, key, ku, ko, etc.).
Lastly... relax. A tense tongue is slow and cause of poor tone.
Get to know your tongue.