The first thing to discuss when talking about professionalism is consistency.
I’ve mentioned consistency before, but it is worth mentioning again. There are lots of great photographers in the world who can take great photographs. But there are fewer who consistently take great photos.
Consistency is the key.
It will not help your reputation or your career if a client were to discover that you have oversold your abilities and that what your portfolio showcases is not your typical work.
As a professional, you should set yourself up for success by knowing your strengths and weaknesses. You should know how you produce your best work. Can you only work with your own lights? Do you need to shoot in your own studio? How much time does it take you to process?
Being a professional is about understanding your skillset well enough to know under which circumstances you can get the results you need, and then doing everything you can to shoot your best work, every time.
Obviously, there will be ups and downs, but if you are prepared to produce your best work every time, you will have more ups than downs.