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,