For me, They tend to appear more regularly in certain areas. Areas near dragon roosts get hit a lot, as do the minor non-walled Holds like Dawnstar or Falkreath. When I fast travel to one of these minor holds that seems to cause a spawn pretty regularly. I would also try the plains outside Whitereach and the western swamplands near the abandoned house where the Dark Brotherhood takes you for recruitment. They tend to appear more in the wilderness or near small Holds and Villages. Try around Rorikstead and the wetlands south of Windhelm as well.
That's really strange. Have you been visiting the Dragon mounds? They look like a big dragon head on your compass. Just off the top of my head there's one south of Winterhold and southeast of High Hrothgar.
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