Here's another reason a line award below the G-line can fail.

You have reserve days lined up until the last day of the previous bid period and the PBS software is only able to build a line with a trip that begins at the beginning of the subsequent bid period. PBS will view the reserve days (even long call) as duty and will deny an award due to FAR 117 constraints. Happened to me last month. Dispute it (drop down menu on the PBS bid page) and you should get an answer as to what happened. Then again, PBS may have made a mistake.