This can be for any number of reasons:
1. Worst case scenario - they are actually holding out for a better offer from someone else
2. Second worst case scenario, the deeds are lost somewhere
3. More likely, they only contacted the solicitor after sale was agreed and it does take several weeks for the vendors lending institution to release the deeds to the solicitor so that they can issue proceedings.
4. Or, the vendors solicitors has the deeds but some ghastly problem has emerged that needs fixing before contracts can issue e.g. vendors title not registered, or registered incorrectly, etc.,etc
5. Or solicitor just sitting on the file because too busy.
If your own solicitor is getting nowhere, ask the auctioneer to get you an answer from the vendor or vendors solicitor with an idea of when everyone intends to complete.
And remember there is no binding contract in existence until such time as the contracts are signed by both parties and exchanged and the agreed deposit has been paid. So, yes, until then either party can walk away with no legal repercussions.
mf