Not having registered will usually only be a problem if you are applying for citizenship. You could try and show that you were not aware of the requirement through no fault of your own, but this is usually difficult. The easiest way of dealing with this is through time. Once you turn 31 (five years after the requirement ends), or 29 (three years after the requirement ends) if you are married to a US citizen and qualify for naturalization after the shorter residency, USCIS no longer cares about the evidence of your registration.