if they have a well formed white shell without candling that is good. if it only looks yellow when it is candled and here is no red spots that is strange. if the shell is hard but there are minor dents in the egg it may just be to dry. try bumping up the humidity by putting a damp/moist paper towel over the egg but not touching it. leave it there 4 a day and see if it fixes your dent problem.
all off my fat tail eggs that have been layed this season i have found them a little late. they got a little dried out and had dents in them but i just made sure they had a higher humidity and they have poped back out now. if your egg is infertile just wait until it starts molding or becomes completly flat befor you give up on it. hope this helps.