Lately I'm not feeling particularly productive. I'm getting things done to be sure, but it all feels sluggish and dull.

What productivity feels like

You know it when you feel it. It's exhilarating. It's a sense of accomplishment accompanied by a desire to do more. This is the defining characteristic of the productivity I'm talking about:
After getting work done you're energized and ready for more.
When I'm feeling my most productive sleep feels like a chore. There's just so much to do and I know I can do it!
Compare this to the more general feeling "getting things done" where there's a sense of completion or even accomplishment but not a desire to do more.
  • After getting things done I want a beer 🍺
  • After a day of productivity I want to do more work 👨‍💻
There's nothing wrong with the former—that's how I feel after doing my taxes for example—but it's the latter that I'm pursuing when working on a new project.

Being productive on new projects

I have some thoughts on what's going on. I think it's the new domain of knowledge I find myself needing that is leading to this lack of productivity. When starting a new programming project it is in many ways not "new" because I'm very familiar with the core activity (programming).
However, when starting out trying to sell a product the domain of knowledge is very different, and that's what I find myself grappling with at present.