I would imagine that any declines are due to the terminal/processing equipment not knowing what to do with the card (before handing it off to the processor) since there are so many configurations of cash register and pin pan hardware and software. Remember that Diners Club cards come in at least two flavors, native Diners Club cards (which are uncommmon) numbers start with 3, while the more common Diners Club Mastercard numbers start with a 5. JCB cards also start with a 3. Discover cards and native UnionPay cards (and some Maestro cards) are the only ones that have a number that starts with a 6. It could be that the terminal balks at the number starting with a 6 and rejects it at that point