With all the good news about how well some drugs are working for some people, it's very frustrating for others to find they're not getting the kind of results they had hoped for. There are many possible causes for drug failure, even when using drugs which have been proven to be effective in clinical studies. Remember that a study reports on average responses. No matter how successful a drug appears in a study, it is not proof that the drug will work in any particular individual.
ALL drugs produce varying results in different people, just as they produce varying side effects.
The first answer is simply because they do. Medical science cannot explain everything and no drug or treatment in medical history has been universally successful. But some potential causes of drug failure are known, and this knowledge sometimes leads to suggestions for overcoming the problem. When it succeeds, the drug may no longer work. Improper use of a drug hastens this process.