Poor service

I app**** for a pension backed loan from FNB (this is the second application) to install solar panels at my home. I completed the required documents and submitted as I have done before with no problems. The process usually takes a week. 10 Days later I call FNB to find out what the progress was and was informed that my application was unsuccessful. I asked the reason for the declined application was was told that it was declined because according to their calculations I am an a deficit ever month. I emailed the bank again wanting to know what they mean and again was informed that I should submit proof of my account surplus form them to reconsider. Again I emailed the Bank wanting clarity as this may impact my future purchases. I called the call center for help and was referred back to the loans department in the bank. I submitted a breakdown of my accounts, they have access to my bank statements and also verification from the credit companies. All I wanted to know is on what calculations did they base my application on. The bank was no willing to give me their calculations and was asked to resubmit my application. If the bank is not willing to disclose what calculations they used to make a decision on my application what would it benefit me to reapply... What really puzzles me is that I am an FNB customer for over 15 years with a clear credit record, no late or overdue payments and receive constant offers from FNB to increase me credit card and overdraft limits. However they are unable to inform me on what they determined that I have a month deficit on my account when I do not have. Very concerning. maybe its time to leave FNB and go to where you are treated like a human and not just a banking fee or interest rate charge.