How big are your crabs? The sand should be atleast double the height of your largest crab. The sand should "sand castle" consistency. Most people try and stay away from fine sands like PFS, The most popular sand for hermit crabs is actually regular play sand, like the stuff you can get at a hardware store. Some people also use coconut fiber or a mix of sand and coconut fiber. Don't use hermit crab sand or cali-sand . hermit crab sand is usually cali-sand, but in the rare instances when it isn't, it's just play sand but costs so much more.
It's also recommended that small crabs have a gallon of room, each and the larger they are, the more space is needed, with Jumbo crabs (tennis ball sized) needing 10 gallons each.
Check the hermit crab food. Do any of them have chemicals or other nasty sounding things? If they do, chuck them. If they don't, they are okay, but you should be feeding them some other things, anyway. Most things you eat are safe, even meat. Most hermit crabs love coconut, apple and seaweed. They can get calcium from things like cuttlebone and oyster shells.
With live plants, the hermit crabs will destroy it. it's not a matter of if, it's a matter of when. I've know of a person who spent 100 dollars on live plants and a year later, not one is still alive. Most people just end up using fake plants.
What type of salt are you using? You'll want to use Marine salt, not aquarium salt or hermit crab salt (yes, seems silly, but the majority of things marketed to hermit crabs are rip offs)
There is some disagreement about using pine in a hermit crab tank. It is known it have insecticide properties, but some people point out the fact it is in their environment. Just keep that in mind when choosing the wood in your tank.
A lot of people use a moonlight bulb, because it heats up the tank, and gives the hermit crab a day/night cycle and also allows people to see them at night.