To be included in the reports tables, such as the Pipeline or Conversion table, a user needs to have sent at least 1 proposal in that period. If they haven't, this could be why they are not appearing in the report. Remember also that they need to have sent/had the proposal accepted at least 24 hours before. So, if they have sent a proposal, but it was the same day as you are searching the report, just check in tomorrow and it should be fine.
Also, by default, only users with the same user role as you have will be included in the reports that you see. For example, if you are a 'User' then only other 'User's' will be included in the reports you see.
You can override this behaviour by adding ?role=all to the URL.