Delta Professional Inc

I am an employee of a very large corporation and Delta supplier and travel often for work. We are supposed to get 'Delta Professional' preferred seating. I am required to book my business travel thru our corporate travel site that is linked with Delta booking via Concur. Every time I book a Delta flight they change my seating to middle back of the plane. I called Delta customer service today and was told there are no exit seats available as previously booked. I check in with My Delta 10 minutes later and there are multiple (5) exit seats available which I need for bad knees. I am disappointed for the last time. Unfortunately, my only other option is American but they never change my seating. If this is how Delta treats it's Delta Professionals, I'll save money for the company and fly American going forward. Avoid!!!!