I’m not going to go into too many specifics but for most map products I’ve worked on, the usual reason that a label doesn’t appear somewhere is because the map data provider hard-codes the potential places where a label could be displayed, and it’s not “everywhere along the road”. You might be zooming into a place where a cartographer chose not to add a label point. Good map software will try to sensibly fill in these gaps but are not perfect. Too aggressive about adding labels and you have the artists and cartographers telling you some areas on the map are too cluttered. Not aggressive enough, and some areas on the map are bare.
It’s maddening getting those bugs saying “I think there are too many labels”, backing it off, then a few days later getting the bugs, “I can’t find the street label next to my house!” Lots of simple-to-whiteboard solutions would work well for your particular neighborhood but look terrible in Manhattan or rural Idaho, not to mention Japan. It’s really not simple.