The way I see it, Solozzo, and his crew, were vastly incompetent. The fact that his men fired like 17 shots and only hit Vito five times is one clue of that. Anyway it could just be that Solozzo didn't look into it very well. He also should have known that Sonny was to hot-headed to make business deals, Solozzo said that himself. And the guy that was in the restaurant, that Micheal didn't know about, didn't do anything to Micheal after he shot Solozzo. Again this expresses their inadequecy.