I don't know if this will help but it's worth mentioning.
lady slippers are in the orchid family, so they need to be treated as an orchid. they don't like too much sunlight.
they like a very fertile soil.
it may take as long as fifteen years for a seed to grow into a flowering plant!

The ones on Manitoulin island seem to like a dryish condition. The few I have found seem to be not in deep shade, but more in marginal light.
You might try lightening the ares areound one clump to see if that improves things. If so, it may be the property is more overgrown than it was. Be careful when clearing. Ram's head (arietinum) does like more shade and often hides under spruce or pine trees.

I also have lady slippers on my property, and the critters get them from time to time. Sometimes they just disappear, only to return a few years later, so I guess the "rest" thing is accurate. Also, when I find them on other properties around here, they are mainly in pine needle duff in fairly dry areas of very acidic soils. I still have the acid soils, but many pine trees are dying out and have been replaced with hardwoods. Maybe that is a problem for you, too? As for light exposure, I have seen them in bright sunlight up in the mountains and in both light and deep shade in the Piedmont, so I don't think that is the cause. They are the most temperamental of the terrestrial orchids...just attended a presentation about US orchids, and the speaker said they are the most difficult to keep going. Good luck.

Their "rest" could be a cost of reproduction. You did mention they had flowered prolifically leading up to this. There have also been studies that indicate leaf size leading up to dormancy impacts flower production the following year. If they enter an early dormancy, say due to drought, they may not have enough stored energy to produce flowers the following year.

As VBain suggested you could let in more light for a clump or two. You might additionally collect rainwater and give supplemental water to a clump or two in an attempt to hold off dormancy and extend their growing season.

I have them growing on my property as well and was told by the prior owner not to let the canopy close too much on them. Mine are growing on a ridge with pine trees. I am hoping they do OK as they put an easement in that area I believe about 5 years ago.

Like another person mentioned, they need to grow in pine fluff. Most of their roots grow between this layer and the soil layer.

I guess it could just be a cycle they are going through... hotter than normal, less or more rain, etc. I do know that most don't bloom each year. Looks like about 10% or so of mine are blooming.

Jeanne

I found these plants growing on private property in Minn. No one knew what they were. With permission I brought two plants in a single clump home. Planted it on the n e side of the house exposed to the a.m. sun. The plant survived and has multiplied to 17 stalks. Has bloomed each year until this spring. Only one blossom. I feel very fortunate to have the plant surviving. Hopefully the plant is resting. I have used bone meal as a fertilize.