If you find that you can't get the lid on your Brita, there are a few things that could be causing the issue. First, make sure that the lid is correctly aligned with the base. There should be a small groove in the lid that lines up with a raised ridge on the base. If the lid is not lined up correctly, it will be difficult to get it on properly.
Another thing to check is whether or not the filter needs to be replaced. The filter should be replaced every two months, and if it's been longer than that since you last changed it, that could be why the lid is giving you trouble. A new filter will fit more snugly in the base, making it easier to put the lid on.
Finally, if neither of those solutions work, there may be something wrong with the threads on either the lid or base. In this case, it's best to contact Brita customer service for further assistance.