There are some unknown factors in this situation...
Often, the offer does not get to the listing agent (you got to make sure the listing agent received your offer). That's why your agent needed to get a written confirmation from the listing agent re. that.
We have seen it happen many times...- either the email goes into spam, or the phone archives the message...So, it makes sense to find out for certain.
If the property was a foreclosure (REO) sale - they could have had so many offers that it would be impossible to respond to all, at least not right away. We are not talking dozens, one property my clients bid for had 73 offers in 3 days, and by the end it probably got even more offers.
If neither is the case though, it is your agent's job to get a hold of the listing agent/his broker (if need to) - but your agent has got to get you the answer.
However, if the offer was too low, and the sellers got insulted by it, they might have choosen not to respond to the offer, and just let it expire. Still, your agent should be able to help you find out all there is to it.
Best of luck in your search,
Hope this helps,
Beachfront Realty, Inc.