Why doesn't my cover fit tightly?

There's nothing you need to do differently except place the cooker where a little splashing won't hurt anything. This is typical of all crock pots. The cover fits loosely because it must allow steam to escape. When water turns to steam, it expands. If not allowed to escape, the steam pressure would explode the vessel. The simplest and cheapest way to let the steam escape is to let the lid lift freely. Some of the steam re-condenses into water as it escapes, and then it sputters and splashes as more steam comes out. They could make a more complex system to control the mess, but then a crock pot would be bigger, more expensive, and harder to wash. I know this might not be the answer you hoped for, but it's a truthful answer to your question so I hope you'll rate it accordingly.