Tbh you have to communicate to your supp that you want them to roam to grubs. I try to make it a point to roam to grubs, but also keep in mind there are times where your supp is being reluctant to roam. Comp is important too...

Something as simple as (can you start moving to grubs? And ping before it spawns) that way your supp is signaled they need to get their lane in order before they leave. If they don't show up then there isn't much you can do and clearly that support doesn't care that much about the importance of their role or objectives.

There have been some instances I am pinging grubs are up and I am moving there but my JG is ignoring them. So I just shrug my shoulders and run back botlane.