Night time cough help

So my step daughter has asthma. She has never had an asthma attack but she is sick very often with colds almost every 2 weeks she is sick. We know how to treat the asthma as my SO has it as well. But when she is sick she will be fine all day but as soon as she is asleep and it starts around 10 pm she starts coughing. We give her cough medicine before bedtime and use a humidifier as well as put vicks on her but it doesnt seem to do anything. Im usually the one taking care of her when she is sick because my SO gets sick so easily where i dont. Plus she comes to me when she is sick because she gravitates more to female comfort when she doesnt feel well. Is there anything i can do to help her get rid of the cough.