Nor is there usually any problem in what people eat. Remedies are not usually antidoted by use of antibiotics or other drugs either. The only factor that can completely antidote remedies and make them useless is human emotion. More likely than not, when a remedy does not work for a person, their emotions could have actually over ridden the effect. A person who has a cold every time something big and important is coming up in their life could actually be subconsciously telling themselves that if anything goes wrong, they can't blame a person who is sick.
Sympathy is another factor.
If there are many people in the office who have had the flu, many people tell themselves subconsciously that they are not better than other people and should get the flu also. Therefore, beyond the use of homeopathics, look deeply inside yourself for the reason for your illness.