Hi and welcome! My name is XXXXX XXXXX I will be assisting you. I am happy to help.
I know of no general restriction for the listing agent/broker to refuse to accept an/or refuse to submit a back up offer. The only explanation I can think of is that the short sale seller and buyer might have signed a short sale addendum to not allow back up offers. However, if that short sale addendum allows the seller to accept backup offers and send those offers to the bank, you might be a in a position to persuade the seller/agent to submit your offer to the bank.
Also, listing agreements may contain a provision that once the seller has executed a purchase contract with a buyer the listing broker has no obligation to continue to present offers to the seller. Also, possibly it has to do with the commission rate/agreement.
Note, it is legal for a seller to refuse to present additional offers to their lender for approval, even if those offers are higher. The lender, however, is an essential party in these transactions so sellers should ensure that they understand the lender’s preferences or requirements when processing a short sale for a borrower/seller. Realtors should remember that the seller is still the legally responsible person so unless their direction is illegal or contrary to the lender’s instructions they should follow their direction. So, it seems that there may be particular agreements that are in place that is the reason why they are not accepting back up offers.
I hope this response answers your question. If you need clarification or have a follow up question, please let me know. Best regards, Damien