Your immune response can be triggered by a specific epitope but provide a less than optimal response. I'm not pulling this out of my ass. Moderna tried making a beta strain specific booster but noticed the original wild strain antibodies elicited from previous vaccination were activated. See also: https://www.cell.com/trends/immunology/fulltext/S1471-4906(2...

I suspect this is part of the reason why we never saw an "update" for the delta variant.