Hi Ravi,
This happens if BOP got failed due to some inconsistency, please check transaction CFG1 in your ECC system, if you are getting some errors for sales orders or other objects you are using in BOP.
In APO transaction /n/sapapo/c3 should tell you if there are issues at APO side.
also check if due to any reason queues are not getting cleared, which might result into issue.
Clear all the queues, Clear temporary quantity and clear the reason for any error message shown in CFG1.
run BOP again, should be through after these steps.
Please let me know if you still experience any issue.
Rgds
Sourabh