Did you moce after filing?
USCIS sent 48 month extension letters to everybody with pending I-751. If you moved and never received one, you may have a problem.
You can request replacement here https://egov.uscis.gov/e-request/displayNDNForm.do?sroPageType=ndn&entryPoint=init
You can also schedule InfoPass appointment and get a I-551 stamp.
Any plans applying for citizenship?