True deep work is cognitively expensive, so the realistic ceiling is lower than people expect: roughly two to four hours a day for most people, even experts. The rest of a productive day is usually shallow work such as email, planning and admin.
That is not a failure, it is the nature of intense focus. Rather than trying to be deep for eight hours, aim to protect a couple of high-quality blocks when your energy is highest, and let the shallow work fill the gaps around them.