How come the dragon disappearing plot was never resolved?

its apart of the overall main plot so most likely around the end of the manga

That is the main plot, but haven't you noticed it has been steadily continued like you said, they aren't just gonna tell you that dragons went to so so and so to do this and that. It starts first episode with Natsu looking for igneel, then it is later continued by him meeting Gajeel who is also looking for metallicana, now we know there is more than one dragon and that they disappear on the same day, then before wendy is introduced, we see two Dragons meet, we confirm for the first time that there are dragons, Igneel and Grandine, and grandine mentions natsu would soon meet wendy, Igneel mentions they must not interfere with the humans and are waiting for the ryuuousai (Dragon king festival). Wendy gets introduced, a third DS, we confirm that Grandine is her dragon/parent, then we see makarov saying fate has made its move that its like porulysica says there are 3 DS in fairy tail, then after a while Gildarts returns and confirms that he meets another dragon, then we reach tenryuu island and finally a dragon makes an appearance to everyone, decimates them and leaves, after the time-skip, we get told why Porulyshuca knows so much about the dragon slayer coming to fairy tail, we find out that she is the edo-grandine, then we find out sting and rogue have been trained by dragons which goes against what igneel said (Which is why i think igneel killed Vicelogia and Skyadrum not sting and rogue as they don't seem strong enough). We then enter the GMG which we are still in, then we realize that the GMG was originally in old times called the Ryuuousai (Dragon King festival), and then we see igneel watching or at least sensing the fight between Natsu x Gajeel and Sting x Rogue. Then we get introduced to the emerald dragon (can't quite remember his name now), then he explains who acnologia is and how dragon slayer magic came about, but he still ignores the reason for disappearance so far. So from my point of view, the main plot has been progressing steadily albeit slowly, which is what we would like, because if it went any faster, Fairy tail would end so i'd rather it went as slowly or even more so. Hope this answers your question, i know its long, i've been told my answers are always long lol

I don't know why the dragon disappearing plot was never resolved but I was reading Fairy Tail and apparently Lucy said something about the Eclipse Gate being related to 400 years ago. I don't really know the years and what not but maybe the Eclipse gate was used to send the Dragons into the future? I don't really know, just thinking about how the Eclipse Gate transfers people through times.