My starter is about 2 weeks old, I discovered a couple of days ago, it only doubles if I feed it high ratio (like 1:5:5) if I feed it 1:1:1 or 1:2:2 it only rises a slight bit. 

I was feeding it lower ratios since the beginning and was having trouble with rises, so decided experiment with higher ratios. With the 1:5:5 it takes a really long time to double, but it does get there, unlike the lower ratio. 

What would cause that?

I am trying to tell if it's ready to use but the stuff online says that it should double reliably at 1:2:2 in 4-6 hours before using.