Hi Abdul,
Try with cheking rule PP. In your screenshot you are using BO, and BO is for Backorder processing.
It could happen that if you have defined a custom rule, then PP is not going to be ok, but it is the standard, so let's see.
It could happen that other order is reserving the inventory.. With Co09 you will see exactly what is happening. You maybe could run COHV or COHVPI and run a new Material availability check (reset the Availability Data).
Kind Regards,
Mariano