There is no binding contract in place until one is signed by both parties, exchanged and the agreed deposit is paid. You're right to be nervous.
There could be a number of reasons : some acceptable , some worrying.
1. There could be title problems.
2. Planning issues.
3. Internal disputes in the Building Company.
4. Holding off to see if any appreciation in price to sell at higher price to you or someone else.
It is a tough world for purchasers when you cannot work out what is happening. I think all you can do is keep pressing for contracts and try adn find out what the reason for the delay is.
mf