I think they have a Respiratory Disease, adding some Garlic and Turmeric into their feed (they are just natural at home remedies.) Keep them in a warm quiet area where until fully recovered.
Reasons:
1. They are coughing
2. "Snoting" Runny nose
Questions
Have they stopped eating?
Do...
I wouldn't give them that much watermelon, that could be the reason they are not laying currently. It should be given in moderation and not as the main part of any diet.