Our understanding was the superseded returns could be filed as long as it was filed prior to due date, and that extensions pushed the due date.  So we filed extensions on all of our business clients. 

A superseding return is a return filed subsequent to the originally-filed return and filed within the filing period (including extensions). https://www.irs.gov/businesses/corporations/amended-and-superseding-corporate-returns

Now we have a return that was extended 3/10 and then filed 3/14.  The client has given us an additional piece of information and we are trying to file a superseded return with the understanding that the extension gives us until 9/15 to file superseded.

This is new so perhaps we are misunderstanding but we can't get the return to go through.