Rendered preview not loading after saving.

I've been working on a project for a while. If I save it and come back to it a couple of days later, all of the preview files load just fine but if I wait a week, none of the preview files will load. I have to render it out all over again. I have limited time to work on this so having to wait for everything to render is eating up my time to work on my project. Does anyone have any idea what could be causing this?

Don't know if this helps but I have several After Effects projects loaded on the timeline.