I'd only ever PM if I knew them well.
Normally I'd just give them space but sometimes I'll drop an encouraging comment on the last chapter they've written. Something along the lines of 'love this story, hope to see a new chapter when you're ready'.
I've had stories that I've left for ages, then someone dropped a comment like that in and it inspired me to write more chapters, so it can work well as a positive motivator.
Well you're a contributor on AAFIL now, so I may hound you for more content!
Seriously though, thank you for all your hounding, I really do appreciate it.