Which part of the brain is not working optimally if an individual is unable to understand scientific concepts and mathematics? I have often observed that while I can understand concepts related to Finance and Literature very easily I find it extremely difficult to understand scientific topics related to Physics, Maths or Chemistry.

If it is due to a certain part of my brain not working optimally, then which part of the brain is it?
Almost every area of your brain would be involved in learning and understanding such high-level topics, so any perceived inability to learn a specific topic is very unlikely to be due to a particular area of your brain not working optimally.

Learning new things can take a lot of time and practice. A lot of times people don't see as much improvement as they would like and this discourages them from continuing to practice. If you can easily understand Finance and Literature, then I believe that you have the ability to catch on with Physics and Math. Just keep reading and trying and eventually you will see improvement.