I think it has to do with the puzzles you’ve completed not syncing up between the website and mobile app. I almost exclusively solve on my desktop, and so when I go to solve on my phone, I think what is happening is that I am seeing the same puzzles from a long time ago that I had already encountered on the browser version. So it will not give you credit a second time, or for solving a puzzle you previously failed. Essentially I think you would have to catch all the way up on the mobile app with with the set of puzzles that you’ve encountered on the browser version in order to start seeing new puzzles on the mobile app and thus, start earning credit for them.
I think it has to do with the puzzles you’ve completed not syncing up between the website and mobile app. I almost exclusively solve on my desktop, and so when I go to solve on my phone, I think what is happening is that I am seeing the same puzzles from a long time ago that I had already encountered on the browser version. So it will not give you credit a second time, or for solving a puzzle you previously failed. Essentially I think you would have to catch all the way up on the mobile app with with the set of puzzles that you’ve encountered on the browser version in order to start seeing new puzzles on the mobile app and thus, start earning credit for them.