I'd say Rick and Dorothea have given some solid advice. There should have been a date on the extension agreement in which the sellers had to reply (accept or refuse). If you still want the home than Dorothea's advice is good to move things along. If you've had enough and just want to know where you really stand, Rick is right. In both scenerios, get a lawyer.
Now, there is one more step to take before you call a lawyer. Every real estate office in Maine has a Designated Broker. Generally this is the owner of the office. It is their license that is at stake here too. Call and ask for the DB and explain your situation, he/she may not be aware of what is taking place. Give the DB the chance to respond and be sure to let them know you'll be intouch with the Maine Real Estate Commission if you are not satisfied with the outcome. Good luck.
Respectfully, Ron H
Buyer Broker, Midcoast Maine
Maine Real Estate Commission (207) 624-8524 for complaints.