When Steven's powers were first revealed, they worked at somewhat of a sporadic rate. He summoned his shield by eating ice cream, and yet couldn't replicate that ability when he needed it in battle later in the episode. He formed a bubble around himself and Connie, and then couldn't figure out how to make it go away. Now, he seems to summon his shield with ease, and has great control over his bubble abilities. A human mind does not seem to work the same as a gem mind, so it makes sense that his powers wouldn't manifest in the same way they did for Rose Quartz. Check out this theory about Steven's subconscious!
