Look at your thumb on the back of the guitar neck. Is it hooked? Is it parallel to the neck? Are you leaning on the edge of the thumb? Is it sticking over the top?
Or is your thumb gently resting perpendicular to the neck, leaning on the pad of the thumb on the upper third of the guitar neck (I have some diagrams below)? This is the proper placement for your thumb (yes we occasionally change thumb placement depending on the technique we are using, but 95% of the time, this is where your thumb goes).
Your thumb should also be place roughly behind the 1st-2nd finger, not stretched out away from your hand.
If your thumb is in the wrong place, change it!
Here are some photos demonstrating correct and incorrect thumb position:
- Here the thumb is too far from the fingers
2. Here the thumb is ‘hooked’:
And here is the thumb correctly positioned: