First please check What is your method returning, i mean query variable? is it true or false seems like not?:
public bool full(string username, string password)
{
var query = from s in db.Subscriptions
join u in db.UserDetails on s.sUID equals u.uID
where s.sPriceABS_ExpiryDate >= DateTime.Now &&
s.sPID.Value == 163 &&
s.All.Value == true &&
u.uUsername == username &&
u.uPassword == password
select u;
return query.FirstOrDefault() != null;
}