Any Clues?

There are several possibilities. One that might be easy to sort out is whether they have access to something poisonous. If they are in an earth-floored run, possible poisons include spilled lead shot (eg from buried shot-gun cartridges). There are other 'poison' options, but try that one for checking as a start. Birds need grit to grind their food. Lead shot (if available) is about the right size for chickens, and it gets worn down releasing toxic lead into the digestion. There have been cases of such deaths in swans when the water level on their lake revealed the place someone had buried cartridges that would have got them into trouble if found.

You could try taking the most recent one to die to the vet- he should be able to diagnose cause of death. Might be expensive though, but could be worth it if you really can't figure out the cause.
Apparently lettuce(not spinach or cabbage,just lettuce) and raw potato are both poisonous to chickens- I don't know if they actually kill them or just make them ill though.

Lettuce???? My hens have eaten lettuce almost everyday since we have had them(March) and they love it!!!!! No sign of illness at all.
I would suggest that you check out the hens that are living. Get into the run with them, and with one of you holding a hen, the other person checking them out. Signs? Firstly, smell. Do your hens smell? Could be maggotts. Secondly, look under their wings and under breast feathers. any signs of creepy crawlys? I'm sure you do but, do you douse them with red mite powder? As well as in the nest box and where they dust bathe, every month put the powder under their wings and around their vent. Red mite is a killer as the mites suck the life blood from the hens. They live in the crevices of the hen house, and according to experts, can be clearly seen with a torch at night...so you could check this out. Thirdly, are your hens yawning? If so, they could have gape worm. hens should be regularily wormed even if there is no sign of gape worm, as these also are killers. I know you say your hens are five months, but how long have YOU had them? and did you buy them from the same place? If recently...within the last few weeks, it could be that they were ill when you bought them. If so, ring up the seller and tell them what has happened. Do'nt be fobbed off either by a casual answer, 'oh this often happens' Rubbish! And, the seller is obliged to give you more hens if you have recently bought them. Unfortunately, there are people out there who will take advantage of new hen keepers. This happened to me several years ago when we bought ducklings and were sold bad duck food. Hens...for self preservation hide their illnesses, because a sick hen goes to the bottom of the pecking order. Also, the healthy hens will attack the sick hen for THEIR self preservation. So we need to check our flock often to ensure they are OK. It's so sad is'nt it? I do hope the others will be OK and you will find the answer to keep them healthy. All the best.

I also feed lettuce to my girls with no apparent problems. I was told by an experienced hen owner that I could give my hens anything except raw potato as it's hard for them to digest and not to give them eggs in case they get a liking for eating their own. It's fine to bake the eggshells and grind them to have back however. Since my lot have been free ranging they eat anything anyway. Even a whole frog! My husband tried to rescue it but Julia swallowed whole rather than give it up.