Well, that's what happens when you use characters who are relatively feared with some of their attacks and can ledge-snap with their recoveries and go to someone like Shulk. He doesn't have those luxuries, and he has a high learning curve on top of that. It's learning to take advantage of those aspects that is what you need in order to master him. He's basically like playing 6 characters in one (if you count vanilla Shulk), none of which you seem to be used to playing Locke. It took me about a month after i picked him up to truly start mastering him, but it'll come to you in time.