The Volume discount clashes with the BOGO offer as the free item is applied towards the volume discount quantity breaks.

Example:

Buy 3 for 15% Off

Buy 4 for 20% Off

+

Buy 3, Get 1 Free

When the Buy 3, Get 1 Free offer is applied, there is 20% volume discount applied as well. The logic should be changed so that volume discounts only apply for items that are full paid for and not for BOGO offers