Maybe she or another chicken is eating her eggs before you get to them? I have recently had a major problem with that. Look for any evidence like bits of shell, yellow splashes of yolk, wet spots that are not poop, etc.
Wow, ok yes i do feed her oyster shells, it is a bit cool up north here, and she always has been a little quiet and lazy, she doesn't appear to be molting, and i don't think they are eating her eggs. Maybe it could be the weather. Wow, great ideas, thanks!
Is there any chance she is hiding her eggs somewhere? We let ours free range and I am always having to search the barn for hidden nests. I know about how many eggs I should get each day and when I don't get that many I know I have to start searching.