What reduces the human body's response to medications even though they are appropriate for his condition? What is the solution to this crisis?

It should not be generalized, it is known that different factors, from genetic to emotional, impact the physical state. Nutrition always depends on which medication and what it is used for and in what condition.

1) Bad or insufficient nutritional status
2) inappropriate prescribing (drug drug interaction)
3) ineffective drug formulation
4) genetic factors
5) Immunologic factors 
5) Individual factors (age, sex,  Body mass index)

Polymorphic nucleotide variants within cytochrome genes play a significant role in influencing both the timing and dosage effectiveness of drugs. These genetic variations impact an individual's ability to metabolize medications, thereby affecting drug response and potential side effects. Pharmacogenomics panels encompassing these genes are pivotal in identifying individuals as poor, intermediate, normal (extensive), or rapid metabolizers of drugs, aiding healthcare professionals in making informed decisions about drug therapy.
 Another factor that I can highlight here is hypoxia. Hypoxia, particularly prevalent in tumor environments, presents a critical factor influencing drug treatment efficacy. In hypoxic conditions within tumors, the effectiveness of drug treatments can be dramatically reduced. The limited oxygen availability in these areas can hinder the drug's ability to reach its intended target, impacting its efficacy and potentially leading to reduced treatment outcomes.

Every individual is different. The effects of medicines may be affected by many different factors. Body size, for example, will affect how drugs are distributed, metabolized and excreted - the bigger you are, the bigger your organs are likely to be, and the faster drugs will be eliminated, meaning the dose might have to be higher than it would be for someone who weighs less. Sex is related to things like how much body fat you have, and hormone concentrations, which in turn may affect drug concentrations in blood and tissue, and therefore how they affect the body. Your genetic heritage affects how enzymes are expressed, again affecting drug metabolism - if your genetic makeup includes a version of a gene that makes more of an enzyme that breaks down a particular drug than others, it will be eliminated faster, and will work less effectively unless you have a higher dose. Other drugs, and food, ingested at the same time may do the same thing, depending on which medicine we're talking about.

No drug is ever 100% effective in everyone. In many cases the dose can be adapted to the individual, something that pharmacometric models of pharmacokinetics and pharmacodynamics are good at. In oncology, it is often less predictable, although better and better success rates are being achieved every year.