Hi Incognito, have you noticed the link at the bottom of the page to the Poultrykeeper website? There's a mine of information there about keeping chickens. Have a look at this page, on redmite, for instance http://poultrykeeper.com/red-mite/red-mites
Although redmite can appear at any time it's not very likely that you have a bad infestation this early in the year, when its still cold, unless your coop was riddled with them last season and you didn't notice and so left them untreated. There are other reasons for hens not laying - could you remind us how old they are, please, and what breeds? And when did you last worm them with Flubenvet? If it was more than 4-6 months ago, a course of treatment may well help them to be able to have more strength for laying if the worm burden is removed. (Good info on Flubenvet and chicken worms on Poultrykeeper as well.)