Re: "But since there was only one next action on the list, I will make no more progress on project A, because it's not on my list of actions to do."
I tend to add a new action when I finish the previous one, or in a mini-review before the weekly review. I also tend to process my inbox to zero substantially more often than the weekly review.
Also, it's definitely fine to just keep on working on a project, even if there's no Action telling you to do so. Many people start with the recorded Next Action, charge ahead for hours or days, and only when they stop do they record another Next Action.
So that's three (three and a half?) potential opportunities for the project to pick up another action. Only if all of those fail is the project left actionless until the next weekly review.
But this is definitely one reason why the choice to have a single Next Action per project, or several, depends heavily on many aspects of you, your job, and your life.
If you do most of your work with your GTD system in grabbing distance, it's easy to keep everything fed with actions.
If you load up a day's worth of actions and go away somewhere for a day to do the work, it's harder. If you load up a week's worth and go away for a week, it's even harder.
If your work follows a predictable sequence, a series of Next Actions may be useful. If the outcome of the current action might affect the next one, creating that series might have been a waste of time.
And this ignores personal things like your tolerance for a large number of actions in your system.
So, It Depends. For me, my system is usually handy, my tolerance for a large number of actions is very low, and the predictability of my work is limited. So I usually have one and only one next action per active project--as well as a very limited number of active projects. In fact, at work I've narrowed things down even further, so that I have a severely limited number of items in a personal Kanban board. I'm considering doing the same thing at home.