Lee,
I took a look at your project mainly focusing on your database structure and the one report you had issues with and not so much with your data flow in your forms. Keep in mind when defining database tables that when you are creating related tables that you have your child tables pointing to parent tables. So in your project players table should be pointing to teams table and teams table should be pointing to league table and I think league table should be pointing to seasons table and for the most part you do have it that way. I am assuming that you have multiple leagues in a season. One mistake I see is that you have the League table pointing to teams and you have teams pointing back to league. And your League is not pointing to Seasons. So one correction would be to change the League relationship field from Id_Teams to Id-Seasons and then remove the id_league field from Seasons. Your Matches should be pointing to teams so remove the id_Matches from the Teams tables. I'm not sure what the purpose of the Teammates table is. Why can't you define all the team players and the team statistics in the Teams table. For that matter if you are only using the Players table to store the player's name, why can't you just store the player's name in the Teams table. I am assuming that you only have 2 or 4 players on a team. It seems like having the Teammates table is making it much more complicated than it should be. Your matches then would only need to point to teams and it's associated players. I did not change any of the structures in the attached project.
The next thing I looked at was your actual data in the tables using SQLiteStudio. I know this data is only test data, but none of your tables had any of the related fields filled in which tells me that your forms are not filling in this data properly which means your report won't work either. As far as your report is concerned, you defined each of your tables in script as separate datasets with no joining of the tables. I changed your print button in the attached project to REPORTSQL instead of using the script and defined the joining of related tables in the Select statement. The report definition was changed to incorporate the Group Headers for League and Teams. I manually filled in some of the missing related data so the report would work. Select the Tuesday Night league when viewing the report.
Like I said I did not look into how your forms are flowing the data. There could be other flaws, but maybe this can get you back on track with your project. I can't spend too much time either, but I'll try to help where I can. Maybe Derek can chime in since it seems he is more familiar with your project.