I work for an in house IT shop. We use a scrum-like system. What we are struggling with now is getting the business owners to follow the process. They all like it in concept until they want something, then the process is for the other people. How do you convince them that following the process will give them more consistent results then just throwing a grenade over the fence and saying, "make it work".
It's probably about time.
Buy-in is called buy-in because there is an exchange just like buying a product. You get the benefit if they follow the process, but in order for them to work with you, they need to get something in return as well. A lot of times executives and higher managers have a lot of things to work on and don't have time to get really involved in the day-to-day workings of things. For better or worse they usually don't have the time to invest in being super involved in your process (like scrum). What you need to do is make the benefit of them spending time in your process outweigh the time they won't have to spend on their 100 other "I need this now" projects.
I would suggest:
It may not be that they aren't buying-in, just that they have other things to do besides have regular meetings with your team. Try to talk with them and see where you get the most value when you're together and focus on that. Try to cut out all the fat and focus on ROI. That should help a lot.