Another reason for delay could be if they want work carried out before granting, or an undertaking to do so signed.
I was involved in one purchase where a staircase needed bannisters installing - the seller had removed them to get furniture upstairs - with three young children in the house I was somewhat surprised at his folly.
I had to give an undertaking to install within 21 days of purchase
(which I did although nobody from the mortgage ever checked up).
Another time the mortgage company refused me a mortgage until the gap where a downstairs wall had been removed and insufficient support put in was remedied - in that case I backed out of the sale.