I have gotten used to issuing contracts and not ever expecting to see them ever again.
Either (a) your purchasers are very sensible people who are trying to organise their loan but are finding the banks very reluctant to deal formally or (b) they are naive/dreamers who have no chance whatever of securing finance and cannot yet believe that this is happening.
It is remarkable to have anyone interested in the property at all so be very careful before you shoo them away. BUT at this stage you need to know are they capable of ever closing this deal or not? If not, then they should stop wasting everyones time and fess up. And, if they can close, when exactly will that be? Can they please sign contracts and agree a closing date so that everyone can just move on?
If they can't sign contracts and subsequently complete , you are likely to find a serious dearth of purchasers out there.
mf