Hi.
Are you sure they are not laying...?
They could have made a nest somewhere, and hidden the eggs there...?
Otherwise, a Summer molt could actually explain the fact your girls would have stopped laying : they would be using their energy to grow new feathers instead of laying eggs, and it is normal - don't worry.
You are not alone in this situation : I own a lot of ducks, and most of them are Indian Runners... but boys AND girls are molting, and so, for the time being, I have what... 15% of my Runners still laying?
Probably 0% soon, sadly...
For the record : it is not even particularly hot in my area, so the heat really won't explain the fact most of my ducks have stopped laying.
(To be honest : it has, in fact, been so cold these last few days that I have to force myself to not use the wood stove... even though it IS Summer!)
...Good luck with your girls.
Don't worry too much : they are taking a break (laying eggs), but they will start again soon.
(It is NOT the end of laying season for your girls - just a break!)
And just so you know : contrary to chickens, ducks tend to look entirely feathered even when they are molting.