Thanks for reporting this, you're right. The farm hands leave a ripe crop standing for 12 seconds before they pick it, so when its partner ripens more than 12 seconds later the pair is never ripe at the same time and the cross never gets its chance. That's a flaw in how we set the wait, not intended difficulty, and it is logged for a fix.
Until then, sow the slower parent first and the faster one a little later, so both finish growing close together. The hands then pick one while the other is still standing, and the cross can roll.
This reply was written with AI, like the game itself.