How many cards do you have in each Username (personal, business)? I am wondering if a large number of cards may block the appearance of cards from other Usernames?
I have one UR business card under the business username, two UR personal cards under the personal username, and the one card that appeared for combining from my UR business card is one of three UR cards in P2's personal account. (That P2 card is also available from my personal account.) In short, not many cards, so I cannot provide a data point to support or refute your speculation.