A new audit finds that the state’s Sex Offender Registry Board has dramatically improved the classification of sex offenders over the past five years but is still failing to consistently collect registration fees. State Auditor Suzanne Bump said in the audit released yesterday that the registry has reduced its backlog of unclassified sex offenders in Massachusetts by 71 percent since 2006. The board has also reduced from one year to 180 days the average time it takes to classify an offender. The audit found that the registry was not collecting the required $75 registration fee from more than 40 percent of offenders. But the report said that five years ago virtually no money was being collected because the authority to collect the fees was in dispute.