Two questions, firstly are you self employed? Anecdotally they don't seem to like them much.
Also what credit limit did you ask for? With this Platinum card, others have mentioned that if you do not specify a high enough amount they will offer you a lesser card with a limit that doesn't meet the minimum limit for Platinum. Others have said earlier in this thread that this minimum limit is $8k, I asked for $10k to be safe and am awaiting the letter in the post to see what I was approved for.
As an aside, when I originally applied for a Gold card with them a couple of years back, I told them I had other cards and that I was barely above the minimum income, they gave me a $13k limit straight up, and preapproved to about 60% of my gross annual income. Go figure!