Check your emails. When you efile you get back 2 emails. The first email only confirms the transmission and the second email says if it was Accepted or Rejected. Did you get either of those? Check your spam or junk folder. The Reject email should say why it rejected.
When you log into your Online account what does it say for each year? Does it say Accepted, Rejected, Printed, Started or Ready to Mail or what?
did you actually efile them or mail them in. if efile did you get acceptance email. if you mailed them, good luck, since apparently you did not use a service to track delivery. anyway go to IRS website and see if you can get transcripts.
if there are no transcripts for those years, the IRS never got your returns. hope you are due refunds because at this point if you owe, the penalties will be substantial
if you were due a refund for 2015, you out of luck. if you didn't file by 4/15/19