I think if the top comment just happens to be like, funny or like, something folks are relating to with the page, some people can just be kind of lazy and like it, yeah I get it-- I think that's just how comment sections with the top comment feature generally work. (Think about Youtube, it's the same way.)
I think the amount of comments you get can sometimes be effected by the time of day you post, but also, literally try asking your readers questions about how they like the page, their day, or their opinion, in your author's comment/update-- that almost always helps with getting comments! People like to be asked things and to express themselves!
Also, you could start trying to use the "author's pick" function for the top comment, picking your favorite one yourself and then folks can make it a little game to see which they can get you to like best? Just a thought.
Good luck!