Well find out the reason for the delay, if it's reasonable (out of town, etc.) then you may not need/want to. If it's simply cold feet on the part of the sellers then there may be other issues you need to look into. Often times a buyers agent or listing agent will have to extend out and offer ideas outside the box to remedy a situation. How to proceed next really depends on the reason for the extension. If they're having trouble locating a property to move to you might find them a top agent in the area where they're looking.
From the sound of what you have provided, the purchase and sales agreement is a fairly standard document, the attorney's both work to add individual addendums or a mutually agreeable addendum. This can take several days if either client isn't readily available to approve any changes in a timely fashion. Weekends don't help. They might just be looking for an extension to sign the P&S for contractual reasons, offers to purchase usually have a time & date for the P&S to be signed, the attorney's would want each client to sign for an extension if the dates were getting close, to keep both parties protected and extend the offer to purchase terms. It's a common occurance if that's the reason.
Hope that helps,