I doubt you'll find any recorded information on it. I've had similar experiences, but with no idea what you're actually trying to do, or possibly doing differently when you're trying harder, nobody will be able to make sense of your problem. Although if you normally just sketch, the looser lines or rendering typically hide a lot of issues with your fundamentals, or possibly with your technique (for example if you never practice clean lines, your lines will look like shit when you try to do them "properly"). There are a lot of factors that go into art, and a lot of different techniques. If you're doing literally anything differently from what you're accustomed to, it's going to look worse even if you're trying hard to make it look good. It could be that simple. Or it could be something else, like you're stiffening up when you sit down with the intent to perform rather than just drawing as usual. Or maybe you try to draw nice without warming up. It may be less noticeable when you're just sketching, but if you're just starting for the day you'll typically perform rather worse than a while into it. Really, it could be any number of things.