Is it a Frequent Flyer Number or Account Number with Southwest

Today, I had a Southwest agent tell me something odd, which seems to be cropping up more and more these days with them.  The gal told me that apparently the number which the passenger and I believed to be her Southwest frequent flyer number in fact is not and she has not been accumulating any miles on her account???  Apparently this has been happening a bit lately and Southwest is aware of the snafu, so they're being good about fixing it and crediting the miles.  I recommend though that before you re-enroll your passenger, to make sure Southwest has their story straight.