Based on consistently feeling as you describe in your opening sentence for over two decades in many different life stages and concept, let me suggest a reframe:

Based on consistently feeling as you describe in your opening sentence for over two decades in many different life stages and concept, let me suggest a reframe: The good news is that "sometimes it's effortless". Consider yourself blessed. For a lot of people that is never true and though they may even achieve some professional success by grinding, it will be joyless and likely not lead to real career satisfaction. Now in terms of optimizing your coding output, don't focus on minmaxing this as if you were an assembly-line manager trying to squeeze out your next ounce of performance. Instead recognize that coding is a creative craft that depends hugely on your state of mind. Focus on finding ways to recognize and harness the energy overall, but recognize you can't force it. This comes in two parts: First, if you are procrastinating diagnose that by practicing self-awareness at all levels. For me it can range from something as simple as eating the wrong lunch all the way up to existential crisis about how I'm living my life, and everything in between. This is obviously non-trivial, but it can be hugely important. A lot of folks mention exercise here—and I agree—but I would broaden it to include ones overall physical and mental health. By addressing those things holistically one establishes the conditions to allow inspiration to strike. Once that is established then you need to recognize and harness the energy when it comes to you. It might be on a schedule, it might not. You might have an understanding boss who gives you carte blanche to organize your own deliverable schedule, or you might need to some stakeholder jiujitsu (up to and including finding a new job) in order to carve out space to make your process work for whoever is putting money in your pocket. The point though, is not to adopt some external productivity narrative (I was brought up with the puritan work ethic) but to instead recognize and optimize for the tao of your own abilities.