That's tricky, but I'd suggest you keep an eye out for where the jays are caching the food. We had a site with exactly this problem, and what we discovered was that the jays were caching the food - either in the lawn or in a tree - and the crows were then immediately stealing it.
We ended up moving sites before resolving the issue, but I hypothesize that the crows would/could have eventually grown comfortable with the feeder and decided to circumvent the jays and go straight to the source. Or, try putting in types of food that are too unwieldy for Jays but attractive to crows (like an egg). That said, we solved the problem with the latest CrowBox design which allowed us to set a weight limit on what would open the box such that jays weren't included - so YMMV!
All best,
Josh