The Syrian Islamist extremist group al Nusra is attracting fighters away from the non-Islamist rebels. This is due to its successes, which are in turn due to the arms it received from countries such as Qatar. The fighters joining al Nusra can probably get better arms too.

Reportedly the US wants to attack al Nusra, but why did the US not pressure Qatar into not arming al Nusra?

I don't think it is possible for any small intervention to enable any other force to win in Syria.