I also use Crazy Angel and if it was fine everywhere else on her body, I doubt it's your gun setting (unless you accidentally turned it during spraying??)
There are a couple of possibilities I can think of - did she Bath/shower or wash her hair just before having the tan? I'm thinking product on her hair could easily transfer onto her neck and back and create a waterproof barrier maybe? Also, it takes a while after showering for the ph levels in the skin to settle down and this can cause the tan to not settle nicely on the skin (I have noticed this particularly around the neck and décolletage areas). If she had used any moisturisers or body oils that hadn't been removed from this area properly that could also create a barrier.
Do you use a tanning mit when spraying? If you do get any runs/spattering you can always dab it in very gently with the mit as you spray so it blends in nicely.