I got my answer.

I didn't get a line because I was "available for a 3 day block of reserve". The junior guy that got the line ended his Sep with 4 days of reserve and couldn't do 7 days straight of reserve.

However now he starts the month with a 4 day trip and they won't be able to use him on the end of Sep reserve block because he'll need crewrest.

They said they weren't responsible for last month when assigning lines!