Kate it sounds like you are doing all the right things, so don't be too disheartened.
Firstly a really thorough exam of your housing for red mite, particularly now the warm weather is upon us. Use a torch if necessary, and wiping the perches with a tissue in the mornings is also a good method.
Next check the houses, try and keep them as dry as possible, and make sure ventilation is adequate. Go easy on the diatom (make sure you are using the good stuff, not the repellent powder) I find a good way to use diatom is mix into a slurry with water and washing up liquid and "paint" into the cravasses. Got any coke left? Then you can use this last thing at night as a spray, then mix up the slurry the next day, a really cheap way to combat/use a preventative against redmite. The cola helps to break down the outer coating of the mite, and the diatom will then work more effectively.
Use a good quality dust extracted shavings, not hay as bedding.
Change their feed to a good quality growers, and add a teaspoon of poultry spice mixed with a couple of tablespoons cod liver oil. As they are moulting their protein requirements increase, the cod liver oil will aid absorption from the gut of nutrients as well as being high in D3. Poultry spice contains minerals, including a little calcium.
If they are on Tylan, continue course for 7 days according to the instructions.
Finally, the problem you have is as mentioned probably due to a respiratory problem, and if you have Mycoplasma, then this is quite quite a complex disease, as although can be dormant for some time in healthy stock, when a number of factors combine, such as moult, weather, stress etc...the resultant effect on the health of your stock can be really challenging, as I suspect is the case here, and also something to consider long term with regard to breeding from your birds and indeed in keeping them healthy.