Unlike some things, this is actually relatively easy to bump up, if you are willing to do some work. But that work mostly needs to be done while your guests are present, not afterwards. I find that it helps if you meet then at arrival (check-in) and departure (checkout). You could also give them a feedback form to fill in. This is a modestly useful indication of whether the guests will be leaving you a review, and if so, what kind of review it will be. People who fill out detailed feedback forms, or even just fill it out in a reasonable way, are likely to leave a review. And if they write nice things on the feedback form, or at least not bad things (the most likely possibility) they will most likely leave a good review. If they refuse to leave anything on the feedback form, that’s a bad sign, and you might want to consider not leaving them a review, and hope they don’t leave one either. I don’t think I’ve ever got a good review from someone who left the feedback form blank.
The reason for meeting and chatting with your guests (assuming you are ok with it and have the time), is that people who have made some connection with you are more likely to leave you a review. Of course, if your place is wonderful and they had an excellent time, they might leave you a review anyway, but an extra push does not hurt.
Most importantly, ask them when you meet them at checkout for a review - assuming you feel comfortable with them leaving a review. You don’t want to ask this if they were trouble, for example. You’ll get some indication of whether they are planning to leave a review from how they react.
I wouldn’t attempt to get feedback from your guests after they’ve left. Unless you’ve really made a strong connection, or your guests feel particularly friendly towards you, it’s too late. For most people, the past is history, and everyone has lots of other things to think about.
However, something you can do for people who you think would leave you a good review if they actually bothered to do so, but who haven’t yet, is to write them a review and then send them a note saying something like - I left you an all 5-star review, I’d appreciate it if you left me a review in return. People will quite often respond to that.
Finally, if you want more personalized advice, I suggest providing a link to your list. Or you could PM forum members as @KKC suggests. But obviously, making your listing public will get more responses.