Block isolation problem

Attached is a pic of my layout track.  The arrows indicate where the blocks are located.  The outer loop is labeled "A" the inner loop "B".  There are other blocks for the sidings, yard but there are not the issue.  I am using a ZW-L to power the tracks.  It's in command mode so I use my CAB2  to power the blocks.   I can power up A and B is completely isolated.   Same if I just power up B.  Here's my problem.  From time to time I run some traditional engines as well as very early TMCC engines. On occasion I've noticed a power drop on the right side of the layout.  So, I decided to add a drop to  each loop, A and B as indicated by the ovals.  I use bus wiring for each power outlet for the ZW-L.  The drop to loop A causes no issues.   However when I add the drop to Loop B, the power separate by the blocks is gone.  Whether powering up loop A or B both power up at the same time...the isolation is gone.  I simply don't understand why this is happening.  Help!