Rye vs Wheat starter

Hello!

The last 5 years I've had a starter based on rye flour. It works great and doubles in size 3 hours after feeding.

Occasionally, I've tried to start one with wheat and it never works. It gets bubbly, but never rises.

I have also tried adding a teaspoon of my rye starter to the wheat starter and it rises a little, but nothing close to the rye starter. 

Does anyone know what I'm doing wrong? Why is it that rye is so much easier to ferment?