Ready; No idle devices are available

I have SBE 2010 R3 and backup to tape library(14 drives) .  I have more 300 servers and more 10 policy

I create policy AAA (full backup(1 time week), differential(every day without weekend), duplicate(full backup in weekend)).

I create policy BBB (full backup(1 time week), differential(every day without weekend)).

full backup created and my job  duplicated not started(in job monitor->sheduled job-> job have status: Ready; No idle devices are available )  if running job with policy BBB.

If i stop job with policy BBB my duplicated jobs running

WHY???