You have a natural pace that fluctuates from day to day.
Some days, you get into flow early and stay in it all day. Other days, you might never get into it. The rest, you're somewhere in between.
You can choose to feel guilty on the days you're less immersed, and perhaps try to force yourself to make something happen by drinking a bunch of coffee, or staying up late.
For me, things got a lot easier when I began respecting and going with my natural pace.