"is this normal or does it happen often? "
All the time. It is the nature of the process currently. The Purchasers have all the ground - the vendors are at their mercy. It is a complete shift in power in the overall process.
All you can do is make sure your Title Deeds are with your solicitor, that they are all set to issue the contracts and that, once they are issued, that you then put pressure on the proposed purchasers to sign and return them.
Until the contracts are signed by both parties - there is no binding contract and either side is free to pull out.
Once the contracts are signed and a completion date is agreed, either party can still decide not to complete, although they are in the wrong to do so, and then it is a question of working out what to do in the circumstances.
My views - the deal is only done when the money is in your bank. Until then, anything can go wrong.
mf