What do you want to do? Force them to complete? Walk away?
Was the Contract subject to their house being ready? Is it a few days here or a few months?
You have options - you can, if the agreed unconditional closing date has passed, ask your solicitor to issue a 28 days Completion Notice. At the end of that time you can opt to force a completion or withdraw. Much depends on what is happening ( and every case is different with different parties, compulsions, issues, emotions etc.,etc), what you want to do and what the Vendors want to do.
Its a help if, once ( if? people do not always make their position clear) you understand their circumstances and you understand the law, you can make an informed decision as to how to instruct your solicitor.
mf