Unpaid fines - Sometimes people accidentally miss all or part of a fine. It is critically important that you order all of your certificates of conviction as soon as you've paid all of your fines to ensure no fines were missed. Further, I have had clients who insisted they paid their fines but the Courts have no record of it. Therefore, always keep your receipts proving payment.
Good conduct criteria - Associating with known criminals, withdrawn/dismissed charges, police investigation were no charges were laid (ie. you were a suspect), police intervention (ie. noise complaints, traffic tickets, causing a disturbance, restraining orders, peace bonds, etc.), may all affect what the Parole Board considers to be 'good conduct'. As a result, they could refuse to grant your Record Suspension if you fail to meet the good conduct criteria.
Generally offensive - Even though you might not be banned from applying for a Record Suspension, if your record is very long and spans decades, the Parole Board could decide that it would simply be offensive to grant you a Record Suspension.