Not submitting a form that was required at the time was a bad move - attention to detail is all-important. Actually, sending the tax return is no longer required (as of very recently) because MOM will now just get the info they need from IRAS. But that wasn't the case last year.
If you want to definitively confirm the status of your last PR application you can do that via the ienquiry service on the MOM website:
https://ienquiry.ica.gov.sg/index.do
IMO, the sensible thing for PR applicants to do is to periodically check that site while waiting for the outcome, and not replying totally on the postal service.
If I were in your position, after checking the status online I'd make a visit to the ICA to discuss the situation face to face. Fundamentally there need to be significant (positive) differences in your circumstances before you apply again, otherwise you are likely to be viewed as wasting the MOM's time (not a wise move).
I'd also suggest waiting for more input from the real experts in this area (Mad Scientist, sundaymorningstaple, etc.).
