Pam had a similar problem with her cat Annabelle. You may want to browse the topic "I need advice!" in this forum.

It may be that Charlie just needs some re-training. Each time he doesn't cover and bury you might want to grab his front paws with your hands and simulate what he should be doing. Do you discipline him when he poops or pees places in places not meant for such activities?

...let us know how you go...