Hi.
Great stress or diseases could cause such a behaviour...
(BUT : the fact your hen sneezes and couchs COULD be nothing to worry about, and have nothing to do with her behaviour.)
What breed is your hen?
During the days, does she have spots/locations to hide herself from view, or to protect herself from wind, rain, etc...?
Are she and her chicks separated from the other chickens?
Are there predators near your home...?
Actually... is there a cellphone tower close to you? ...5G? Even 4G...?
Or are there wind turbines...?
...Did your check if your hen is wounded...?
Or do you think she could (also) have vitamins deficiency?
(Did you deworm her?
And treat her for lice and mites?
...If so, what products did you use, exactly?)
...You don't know what her problem is, but you could already want to diversify her food, and simply give her food she likes... a lot!
Give her some fruits, for example... or fish (especially salmon)... or wet cat food - chickens love it...!
Could really help to de-stress her, simply because she will enjoy eating something that tastes good and she does not eat everyday...
I would personally mix some curcumin, garlic, ginger, and/or peppermint in her food : they are renowed to fight against stress, and can't hurt if their dosage is not extra-abused...
(And if I was not allergic to essential oils, I would put a mix of water and - just a bit of - Peppermint Essential Oil in her coop in the evenings (just before she is going to sleep), everyday for several days - 14 days MAXIMUM.)
...Would you have a picture of your hen?
Could always help to determine the problem, if people see something on your picture you did not notice...