Different Banks have different procedures for submission of Offers on a Shortsale or an REO;
it may be that the Bank instructed the LISTING Agent to only submit the highest and best offer, or offers. Or they may have given him mimimum guidelines for each Offer. This would not be public knowledge.
Yes, it might take months to see the results, and you knew that going in.
The Agent deserves the benefit of the doubt; this is not a disreputable profession.
Possibly you should move on.
You may ask for some type of written proof of the submission, such as a Welcome letter...however, he is not required to provide this confidential information.