Starter Got Weak. Why?

Hello,

I've been making most everything with sourdough starter lately, and I'd been making that starter progressively stiffer.  It worked really well until I dropped from 60% hydration to 50%.  The first few loaves from the 50% starter were fine, and the noticeable extra-sourness encouraged me to keep the starter 50% stiff.  However, my loaves soon became dense.  I thought it was other factors, but now I believe it is the starter, as it seems low on gas, and the 'alive' feeling of properly fermented dough.  

I've mixed some of the stiff starter with fresh flour and water in a ratio that will bring it back to 100% hydration.  I used about 20% rye/whole wheat in the flour feed.  The result was not as bubbly as I would have hoped.  

Any thoughts on why my starter got weak when I tried to go stiff.  Also, any thoughts on how to reinvigorate the starter I have?  

Thanks in advance.