Just want to agree with Chris, the calcium/phosphorus balance (as well as a few other possible minerals that can upset calcium absorption) is really crucial to good eggshells. Even something as simple as adding a cupful of milk or yoghurt to a daily mash can throw the balance out, and gradually cause thin shells.
As well, lack of vitamin D can do it (as Chris explained).
A third issue can be when birds are fed too much calcium (eg. layer pellets) when very young; this reduces their ability to absorb sufficient calcium later. Hopefully that isn't the case here.
Worms can also interfere with calcium absorption; indeed any digestive ailment (e.g. algae in the water causing gut lesions; any forms of enteritis) can interfere with shell production in this way. It may be an idea to worm them just in case, and make sure all water sources are clean.
Lastly, high volume commercial layers have been bred to the limit of their ability to absorb enough calcium to make the number of shells they need to make in a lifetime. If your birds are aging they may need a break (e.g. feed them only wheat for a couple weeks to make them moult and go off lay). I would think about doing this anyhow, if the soft shelled eggs remain stubborn, in case the break puts them back on the lay with better shells.
Back to shell grit. They will eventually start taking the shell grit, especially if you sprinkle a little on top of a mash. That way it's not mixed in, so they're aware they're eating it, and they soon develop the ability to take it ad lib.
good luck,
Erica