Your scores are pretty much just borderline for the one card you have. I would let things age and get the score up into the high 600's to low 7's before trying for a second. I don't have a second Discover, but I've seen enough approval posts to know those are just barely getting in or they lower now than when you got your first card?
OP, reviving this because my wife has got this same message and her FICO's are all 710+. I did have her as an AU but removed her a few months ago but it still shows on her account.
Were you an AU on anyone's Disco?
