Speaking as a practising solicitor, I seriously doubt that any solicitor would lie and say they had lost a bank draft so that their client could drag out the closing date. Especially as the vendor is, until they sign the contracts with a specified closing date, quite entitled to name the closing date in any case.
No, I'd agree with CCovich, its more likely the bank draft has become misplaced in either your or the vendors solicitors office. It doesnt happen often, but it does happen.