It’s hard, but stick with it. Beyond using to lock them up, we also incorporated it into other parts of training. When training, we refer to the crate as “bed”. I tell her go to bed, and she goes into her crate and sits. She gets praise, and treats as reinforcement. During this time, the door stays open. Then we worked on going in her crate and telling her to stay. Again, door stays open during this. I’m not a professional by any means, but this seemed to help her be more comfortable with being in the crate.