Here's the thing.
When I first started playing, dragons would show up ALL THE TIME at random whenever I was out wandering or even when I was in a town.
Now, there's NONE to be found that aren't at roosts in my game. And I wander A LOT.
Really, the same thing happened to me.
Maybe it depends how many souls you have on you. For awhile in my main game, they wouldn't leave me alone, I literally had 15 souls in waiting.
Its still just random.
My two other characters I'm playing, they are much more rare to start. The fact you had them all the time and then none at all is just how "random" works in video games.