It is just because your assignments are closed.
Just ckeck the "show closed" checkbox (wich is automatically checked when item is closed) to see the assignments.

Hi Babynus,
I misspoke. 
To reproduce the case, you have to create a meeting, prepare it, carry it out and put real time on the participants. Then close this meeting. 
Then reopen the meeting, and when you want to edit the participants' time again, it's impossible, as the table is only displayed in the closed state. When you uncheck the box, the table disappears completely, so it's not possible to modify it. 
I had to change the toggle status directly in the database to be able to make my modifications. 
 

Of course behavior is as expected.
Closing the meeting closes the assignments (same behavior occurs to activities)
Reopening the meeting does not automatically reopens the assignments.
To be able to close the meeting, you have to set all left work to zero, so assignements should be ended.
Moreover, when you reopen the meeting, we cannot know wetjer the assignment was manually closed (and then should stay closed) or was automatically closed when closing the meeting. So the rule is to keep the assignments closed.
Just re-open then if you need to (since V10.4 you can update many assignements in one step wil mutiple updates on new assignment form)
 

Hi,
Thanks for the reply. 
What I want to do is re-open the affectation of a meeting or activity, delete the real activities then could edit the line in order to change the ressource's cost. 
But if the line doesn't appear whand this mission is not closed, it's impossible. 
 

Just check the "show closed" checkbox.
Then you'll see assignments and will be able to reopen them