Even though your dealings with them may be exemplary they will still automatically do a credit check before issuing a cheque book, in part to see what's changed since the last one / is it increasing thier risk of non-payment. What's your credit history like? Do you have any outstanding debts? And remember 'bad marks' sit on your file for 6 years.
If you have no idea why you've been declined, visit them. Go in and ask for a private room consult and then ask the staff member to contact lending directly and ask why you were declined. You may both have to hold for a while but you should get more info. It may be that by talking to lending and answering new questions that they review thier descision...it worked when I worked there!! And avoid lunchtimes when they are overworked and stressed...
Good luck