Why would a bank take longer than scheduled to certify a home buyer's checks?

Our closing was held up yesterday because buyers came with the wrong type of checks! They wanted to wait till next week. Their lawyer then wanted to front the money for them. We said "no". (They have had 3 extensions on the closing date already! Each time finding an excuse that they don't have their mortgage yet. They had already gone ped the deadline of the contingency before they even notified us that they didn't have a mortgage! )They called us and said that they would finalize the paper work and get the checks today. We agreed to wait till today for bank to certify checks. It was scheduled for 12:00pm. Our lawyer called and now says the checks won't be certified until 3 or maybe 4 pm. Why would this happen? Is it possible there is some type of mortgage fraud being attempted?