My hearing test is normal but I have trouble understanding speech. Why?

Maybe aphasia. I am glad you had your hearing checked first as that would be the initial thing to look into. If those tests are normal, then how your brain processes speech. Some strokes can affect the part of your brain that allows you to hear and speak properly. Problems are called aphasias. Also, even with normal hearing, some people can be distracted if trying to hear with lots going on. Get checked out.