Tough Stew Meat for a Carbonnade
It sounds like you did everything right except you didn't cook it long enough. Pre-cut stew meat is often from tough cuts, and can sometimes take a long time to get tender. I would suggest lower and slower next time. Also, maybe you didn't rest the meat after cooking, I would suggest resting at least 30 minutes but give it an hour if you can. Try giving yourself 4 and a half hours from start to serve, 3 and a half for cooking at 150C and 1 hour for resting, that should give you a more consistently tender result.