I'm a certified buyer representative and can help you with the purchase of a home. I've helped + are continuing to assist buyers, including ones with a pre-approved FHA loan, to move forward in the purchase of their dream home.
Don't ever expect a listing agent to work in your best interests, as their fiduciary obligation is to their client, the seller.
I would try to find out the reason from the agent why your offer was declined. Is it because:
(1) the house may not pass FHA guidlines and the owner is unwilling to do the repairs or
(2) It's an older owner unfamiliar with a very legitimate product and cannot grasp the concept of the sigificantly lower down payment option. In that case it's really up to the listing agent to educate the seller unless the listing agent is also clueless on how FHA works or affraid to upset the seller.
Most likely, the agent for the seller knows of issues with the home that would present problems for the FHA appraisal/inspection that would ultimately be performed.
FHA has a set of "minimum" standards that the property must meet to qualify for the financing. If there are any major issues with the property, i.e. structrual, plumbing, etc, etc. the property may not qualify for the financing, potentially killing the deal - or at least forcing the issue to be remedied prior to approval. If the agent knows of these problems in advance, they will simply not accept FHA to avoid the deal falling through as a result of the property not meeting the FHA standards.
Hope this helps!