Strange AlphaPix4 Hardware Behavior

[ibanezsvo]

*solved w/ null pixels.
Background, my AlphaPix4 controller is hooked up using only 2 outputs, and I only use HC Brilliant Bulbs on it.  Out2 feeds roof outline - about 280 bulbs w/ power injection.  Everything here works fine.  Output1 hooked up to three window frames, each in series from Window1 then 2 then Window3.  Each frame is 21 bulbs each so I'm not experiencing power issues (as proven below).  Window 2&3 are right next to each other and connected via a short 5' jumper (HC's 3/C wire).  The intended hookup is: Out1 -> Window 1 then a 15' jumper to Window2 and then short jumper to Window3. 
I believe this is a hardware issue because I experience the same results in Test Mode on the controller and using xLights output.  Here is the problem and some tests I've run and their results
When connected as desired, Window1 works fine, Window2 two gets the first 2 or 3 bulbs that act right then the next bulb flickering, then nothing - including Window 3, nothing. 
Windows 2&3 hooked up alone, no problem.
Windows 1,2,&3 hooked up with short jumpers between each - no problem.
Window 1 hooked up (working fine) then to test power concerns I connected a new brilliant bulb string 50 bulbs/10" spacing and all bulbs (total 71) lit up and worked as expected.
I did some impedance tests on each conductor going from end to end (bulbs and controller disconnected) and got about 1ohm on each wire.
Same wire used in the 5' jumper as in the 15' jumper and I've replaced the 15' jumper with a different piece and I've even run two pieces in parallel.   
I can't figure out what to do next and have no idea what is causing this. 

[markrvp]

I answered this on another forum, but I'll summarize here:

The Alphapix isn't the problem.  You've already proven that everything works correctly when you remove the 15' cable between Windows 1 & 2.  That tells you right there that either the cable or the distance between window 1 & 2 is the problem.   

Here's the easiest solution:  You're only using 2 outputs.  Reconfigure so that you can run Windows 2&3 off of SPI 3 on the Alphapix. 

[ibanezsvo]

Thanks for the response. Usin out 3 would have solved the problem but that wasn't a long term solution, cause I was bound to screw this up again as I added more to the show. I didn't understand the concept of null pixels and why they were needed. I put in two null pixels and that fixed my problem. Thanks again!