In the most extreme bad runs you could go 70 to 80 searches without one dropping, so I would say there's something wrong if your going half a year without one. Alternatively, its possible to get 4 out of 5 runs...
This topic was discussed a while back and someone suggested giving the artefact search a break for a day or do, sending them out on treasure or adventure searches for a bit. Not sure I'm totally convinced, but if I were desperate I might try this...