UK Landlord Forum / Life

Hi
Looking for thoughts and advice on the following.
I've recently took steps to remortgage onto a BTL mortgage following letting my property out. Through a broker a mortgage offer was successful but have now been informed this isn't the case due to my tenancy being non assured.
Why would this be unacceptable compared to a AST??? I

Mortgage companies don't like anything out of the ordinary, and anything other than an assured shorthold tenancy is very unusual these days. What sort have tenancy have you set up?