It is a very common problem, and is much more obvious when you only have a couple hens. I have about 20 birds and they all take turns laying and taking breaks, so I don't always notice the ebb and flow that each one has.
If they are older than 2 years, it may be natural to slow down. If they...
Are their combs still bright red? I would check for external parasites, and maybe think of worming them. Secret nests are something you need to rule out, too.