Somethings wrong with Montego Karoo Senior 20Kg

I purchased Montego Karoo Senior and after going through the change food process my 10 year old Retriever was quite happy with the food but that didn't last for long. He started throwing up followed by diarrhea. He would eventually sniff the air and walk away from his food. My dog is food driven so this wasvan immediate red flag. I smelled the food and it did have a strong stale smell although its kept in an airtight container. I tried another brand and he wolfed it up and was back to the happy dog he has always been.