The disconnect between Sales and Operations in manufacturing companies is so common that it is almost a cliché. I’ve experienced it myself and I’ve met plenty of people at other companies going through the same struggles. Maybe you see some of this in your own company.
Sales perceives Operations as incompetent, unable to deliver the right products at the right time. Operations perceives Sales as unpredictable, overly demanding, and unreasonable. And customers are unhappy because they're being shorted.