Up until a couple of years ago, someone could be in a 100 year flood zone and obtain insurance through the National Flood Insurance Program very easily with no documentation. You could obtain an Elevation certificate that gave the actual elevation of your home in comparison with the base flood elevation of the flood zone your home is in. If you home showed to actually sit above the flood elevation, you could qualify for a discounted rate.
Now, an elevation certificate is required by most, if not all companies. Flood insurance is typically backed by the NFIP and should have the same requirements regard less of the company you use. The elevation certificate will generally cost $200 - $300 to have performed by a licensed surveyor.
The only other reason I can think of that a carrier would reject your purchasing flood insurance coverage would be the number of flood losses that have been paid out on your home. Generally, if you have 2 flood losses at that home, you're then refered to be insured directly through FEMA and the NFIP.
Your agent should be able to help you with these answers and obtaining flood coverage. If they are not, find another agent, because you should be able to obtain coverage. It might just cost a little more.