​I have explored over 100 times, but only obtained 1 star fossil. it's very difficult to get  when you put 4 items into one location!!!

​I encountered the same situation and thought it was my own bad luck.

Same here.  Over the course of lots of explorations (not sure exactly how many, probably somewhere between 30-40) at that location in two different museums, I have only found that exhibit twice, and only at Memento Mile.  I'm not that far into the game, but I haven't run across anything yet that might increase the chance.