Fertility troubles

I bought five quality heritage wheaten Marans last spring, four pullets and a roo. They started laying in late summer. One dropped dead a month ago for no apparent reason. One consistently lays bloody eggs, I didn't set her eggs. Two were laying well a couple weeks ago when I collected and set 17 of their eggs along with 20 others. The wheaten Marans roo fertilized all. Not one of the wheaten Marans hens eggs was fertile, yet 15 of the other 20 were. So those two girls are the problem. Is this common? Could their eggs be fertile later in the season? Or are they no good to breed and I should move on? I don't know how normal this is. Any advice would be much appreciated.