So according to your history, you had seen 2 symmetrical points on your shoulder blade that do not tan !!!
I think its due to reason that , those two points were more in contact with your tanning bed
, to that two points have been differently treated by the tanning bed !!!
There is no need to worry about that, just try to change your positions in the bed (if possible) !!!
hope my info helped you
Dr Hari Om Chandrakar