7i85 5V jumper confusion

I seem to have some pretty crappy parallel cables connecting my 7i76 and 7i85 daughterboards to my 5i25. By the time it gets out to the encoder connector, I'm only seeing about 4.2V, at which point one of my encoders refuses to work. While I await decent quality cables, I have a 5V supply connected to the P1 header. I'm now seeing the full 5V at the encoders but the encoders are not being seen. If I remove the external supply and swap the jumper, they come back.

What's going on here? Do I need to swap the jumper on he 5i25 too? And run the 7i76 from an external supply too (both or nothing)?

Hopefully there's a simple answer.

Thanks