Drives are in use in storage unit

I have three policies running which are using the same STU. Three jobs (including the parent job) are active from the first policy, three (including the parent job) from the second policy. However, the one job from the third policy is coming in queued state.

MPX setting at the STU end is "7"

The MPX setting at the schedule end for the third policy of whose job is coming in queued state is set to "1" which is no multiplexing.

I have tried cancelling the queued job, changed the MPX schedule setting of that policy to "2" but still its coming in queued state. The job state is "Drives are in use in storage unit".

Kindly let me know how can I correct this. Thanks in advance.