Hi, Lynn, does your purchase agreement and/or addendums call for certification by a licensed professional septic contractor? If it does, you might check the contractor's license. Call the contractor and clarify exactly what the certification covers in detail. File a complaint with the contractor's licensing authority if you're not satisfied.
If you're still within your inspections period, or otherwise agreed to, you could pay for your own additional septic inspection.
There may be other options; consult with your agent, your broker, and your attorney.
Hope this helps!