Thank you very much for your response (and your patience)
1.- Thank you for teaching me the difference between "group mode" and "grouping". This clears up a lot of things.
I am confused by your scenario. If you want to check the attempts made by your students on a Lesson, why do you go to the gradebook? This is not necessary, just go to the Lesson and click on the Reports tab.
2.- Checking first the gradebook is just because of the way I work. I glance first at the gradebook to see how many students have grades and then I go to the activity. I'm aware that I can check the attempts directly from the lesson, but this doesn't solve the problem, I still see the message of no attempts made when the lesson is available just to one group (even though all students in the group selected made attempts). As an end user, and having learned the difference between group mode and grouping, when clicking the tab "Reports", I would expect to see the list of all the students of the course that have made attempts, irrespective of the group they belong to.
Please, can you check that if you set your Lesson to be available to the members of one of your course Groupings only that Lesson is indeed visible when logged in as a member of that Grouping?
3.- Yes, the lessons, quizes and resources are visible just for the members of the group chosen in the grouping setting (provided that the checkbox available for group members only is also checked), but this holds true only for the main page of the course. Students of one group can see the activities of the other group in the gradebook (including lessons), and this creates a wealth of confusion (and a miriad of messages in my mail box), because students see that there is an activity going on but they cannot access it, so they fear to miss the grade. In my humble opinion, what they see in the gradebook should be congruent with what they see in the main page of the course.
1.- When a lesson is made available for just one group, clicking the tab Reports in the lesson page, gives a message of no attempts made, even though members of the selected group have attempts.
2.- When grouping is selected, visibility is correct in the main page of the course, but in the gradebook, members of one group can see the lessons of the other group. Quizes have the same behavior.
Hope I could help you and thanks again