Don't be discouraged! I was actually told the opposite-that a birthmother may actually prefer another child in the home because then there is a guaranteed sibling. Our wait for our second child was actually only 3 weeks.
I think in your situation it is more the fact that it is a bio child that is your first vs a child through adoption. Not that there is anything you could or would want to change about that of course!
Isn't that frustrating though when agencies do that to you though with their wait times?
We were going to sign with a rather large and well known agency in the South that stated a less than 6 mo wait. Well, the day before I was about to send off all the paperwork and money, I received an email from a friend that just terminated her contract with them because the wait had now gone up to 18-24 months. I was furious to say the least so I do know slightly where you are coming from.
Good luck to you and I just pray that you can have another child in your arms very soon! Best wishes-Sarah(mom of 2)