I have a case list with a follow up form where the users insert a number and a calculation add it to tally.
If a user start filling the follow up form, and at the same time another user submit data, the tally dosent update for the first user.
There is any way to resolve that problem?
That's unfortunately a very tricky problem to solve, and I don't think we have a great answer to it. You can make this form as short as possible and train your users to sync immediately before and after using it, but that's still not foolproof.
If you can somehow use a different tally for each user, that would be safer.
If the total value isn't needed within CommCare, you can also instead get the individual updates from forms and sum them later when you export the data, rather than keeping a running total on the relevant case itself.