Will my chicken start laying again?

I had two chickens (Charlotte - Gingernut Ranger, Harriott - Miss Pepper Pott about 2.5 years old), neither have them have layed since early February other than the very occasional shell less egg a month or so ago. One (Harriott)has now died after what I thought was a case of sour crop.
1. They both moulted, but have fully regrown their feathers now
2. They are fed layers pellets , but have other treats such as vegetables, seed, meal worms, raisins etc
3. They were both are alert, tails up and were eating and drinking normally
4. They free range around my garden
5. They have access to grit and oyster shell.
6. They have been regularly wormed with Verm X, but incase this wasn't working they have also recently been treated with Flubenvet.
7. They have both had a bit of diarrhoea, but are also producing normal poos, recently with intestinal lining in  and what look like bits of egg
8. They are having poultry tonic in their water
9. They have recently been given some live yogurt, as I read this might be helpful for them

Due to the lack of eggs we recently introduced a new point of lay Chicken (Daisy - Amber Star). Now that Harriott has sadly died I am worried that they may have something more serious than I thought. Is Charlotte likley to ever lay again and is she and Daisy likely to be at risk from what ever got to Harriott. Harriott seemed perfectly well until a day before she died and went down hill very quickly. Both the others seem perfectly well.

I would appreciate any advice you can give.