I'm trying to make some report and I'm interested in the value of field "SolutionInMin". As I can see from other posts this value should show how many minutes spent for ticket resolution (from ticket creation till ticket closing). I've made several tries but I was not able to see a report with a ticket having this value different from 0

What I've done:
- defined a custom calendar
- defined a custom service
- linked every Customer Users to the service just created
- linked every queue to the custom calendar just created
- created a SLA linked to custom service created and to the calendar created (Escalation - solution time 30 mins)
- created a custom report (object: Ticketlist, fields: almost all, restrictions: none)
In my OTRS I've got many tickets and some of them are closed (successfull/unsuccessfull). But when I try to download the report via CSV every value of the column "SolutionInMin" and "SolutionDiffInMin" is 0

What am I missing?
Any help appreciated.
Luca