So the storyline involving this quest can understandably be a bit confusing (as evidenced by the earlier posts in the thread!)
If you haven’t already started any of the zone-specifc chains after Where in the World is a Lost Blue Dragon? you’ll need to click the Dragon Locator Orb and do that.
If you can’t pick the next quest from there, chances are you’ve already started on one of the chains, but left it partway through and you’ll need to manually travel back to that zone and wrap up the full chain.
More info here:
If you’re not sure where you left it off previously, you can find more info about the specific quests to check for here:
Best of luck and I hope you can find your way back on track!