BLUF: Any advice on setting up tools/project selection for shared resources?

We have about 800 people at our site. The issue is that we do not do a traditional "Project Selection" workshop as I had in my past. I was informed that we did that once a few years back (before I joined) and the priorities established were trumped by another project a few months later.

I am curious to know if others have had success with tools, meetings or other ways to initially allocate shared resources to the projects we do. We also have issues where commited resources to be available on June 1 but come May 24 those resources are suddenly "Not available".