有.此复合句可拆分为以下几句.
1:I knew the reason.
2.I failed the exam.
3.I didn't know about the knowledge exactly
整体意思应该是：我知道我没有通过这次考试的原因是因为我不能精确地理解知识.
I knew the reason that I failed the exam （1,2整合语法没错误,但感觉别扭）
但was I didn't know about the knowledge exactly有错误,was不能直接+句子,应该在was后面+that+句子.另外about可删除,显得多余.
经初步修改后,可改为I knew the reason that I failed the exam was that I didn't know about the knowledge exactly.
但我认为这个句子可写得更地道.可写为：I knew the reason why I failed the exam was that I couldn't understand the knowledge exactly.