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