Using Probiotic capsules to grow a culture

I tried to produce yogurt from an over-the-counter probiotic, by emptying the capsule into a sterile container of milk and setting it in a warm location for a couple of days. I've had great success with this method using store-bought yogurt and kefir as starters. I wanted to try the capsule since it had more diverse bacterial strains.

It didn't work. The bacteria didn't grow. I assumed they were dead, but someone from the manufacturer told me that their strains don't grow, even though they're alive and active.

This seems strange. How can live, active bacteria not grow??? Can anyone explain this for me?

And, can anyone recommend a good source of diverse strains that WILL reproduce? The store-bought yogurt and kefir aren't bad, but I was interested in maximizing diversity.