I wouldn't ask them more than once and maybe not even ask them directly, but rather something like
I would be glad if you left a review on our ... page!
This way, you don't come off as nagging and they feel not obliged to write one. You are just saying, that you would be glad and don't demand them to do that. This way, they may actually be a bit more inclined to do so, because they don't feel like you annoyed them. It's short and they don't need to respond, so even less awkward for them.
So they were willing, but didn't. Usually, that's because they forget about it or think that it is too laborious. So the somewhat un-IPS-like solution would be to make it as easy and convenient as possible - e. g. a good site with up to date information, that they would check anyway before visiting you (e. g. informing people about general open/closing times or some special cases, when your place is closed, menu card etc.). This costs some effort and may take some time, but people learn that it's a reliable source of information.
This would give you an even subtler approach, like
Check our ... page for up to date information, where you can also leave feedback, if you like.
Ready with business cards containing this information also.
The drawback of my approach is, that it is not especially forcing (but then it's hard not to appear nagging).
My qualification is that of a customer, not of a business owner.