Do you, as plaintiffs, have a lawyer representing you in Case B and in Case A also? Otherwise, you may want to seek one. My opinion is that the lawyer (unless he/she is a defendant in the case) cannot be served (for instance in case A even if he represents the same people in case B) unless the lawyer has "agreed to accept service of process for the defendants" and files a "Notice of Appearance and Waiver of Service" on behalf of the Defendants in case A.
So, it appears his "notice of defective service" may be correct.
You would have to either speak with your lawyer to see the reason why he/she wanted to serve the summons/complaint or petition on lawyer for case A AND/OR try to find each defendant's actual location to serve him/her directly by personal service of process (if that type of process is required).
Note that service of process rules are a bit complex, specially for the type of case you have, the defendant type, and what you are trying to accomplish.
Alejandro R. Lopez, Esq.
Law Office of Alejandro R. Lopez, P.A.
4465 Edgewater Dr.,
Orlando, Fla. 32804