Students sometimes say "I find it hard to hit single notes in the wide mouth position"
You should stick to the deep & wide relaxed mouth position. Here's a trick which would help you in getting single notes - Put the harmonica in your mouth, with a deep relaxed embouchure. Your mouth should cover 3-4 holes on the harmonica. Now, start blowing. You'll hear a chord. To get single notes, start tilting the back of the harmonica UP. You will notice that at a certain angle, your lower lip will block off all the holes except for one...and you will hear a nice, single, blow note! This is how single notes work- Your lower lip is responsible for single notes. The lower lip channels the air to go through only one hole, and blocks the off the side holes. Practice doing this several times. Once you get good at playing single notes, you will be able to do it without tilting the harmonica so much.