Perhaps you completely drop any trace of taking the easier content even remotely seriously so then you're prone to make silly mistakes. Or you're being distracted so much by something (messing around in voice chat maybe?) that you cannot pay enough attention. If what you're saying is true it's not that you can't do easier content, it's that you're not approaching it in a similar way to harder content. I'm not saying you have to be ultra serious about it, but it sounds like you need to give it more attention than you have been. Clearly some part of you just switches off in easier content, for whatever reason.