I don't do Silkies but there are many things that could be working. Fertility is not the only thing that can cause clear eggs. High or low temperatures and rough handling are a few that can especially affect shipped eggs. Those things are not in your control and not very much in control of the shipper. The reason I put a little responsibility on the shipper is the shipper controls how they are packaged. Other than packaging the shipper has no control over how they are handled during shipping or after you receive them.
Sometimes some heavily feathered breeds, like Cochin and Orpington, can have a problem with fertility. The thick feathers can block the rooster's target or delivery system. It does not happen every time with every chicken in these breeds but it can happen. I believe Silkies fall in this category of heavily feathered. Some breeders get around this by what is called vent trimming. You either pluck out or trim the feathers around the vent that may be blocking the male from a successful attempt at mating.
There are other things at work too, like health, nutrition, and age of a flock or age of the eggs, but these are not specifically Silkie related. They affect all breeds.
I can't tell you what your problem is, just that there could be many causes. Shipped eggs are often harder.