It may be a highly lipophilic drug, and so easily uptaken into body organs and tissues.Thus, the volume of distribution is higher.
Transfer of drug across membrane is dependent on the concentration gradient, as well as the lipid-water partition coefficient of the drug, as well as the membrane surface area.
The greater the partition coefficient, the higher is the concentration of drug in the membrane and the faster is its diffusion.
Many drugs are weak acids or bases that are present in solution as both the non-ionized and ionized species. The non-ionized molecules usually are more lipid soluble and can diffuse readily across the cell membrane. In contrast, the ionized molecules usually are less able to penetrate the lipid membrane because of their low lipid solubility.
It may be due to the route of administration of the drug. Other routes of elimination could include loss of drug in saliva or sweat, secretion into the GI tract, volatile elimination from the lung, and metabolism at other sites such as skin. THis would decrease blood concentrations of X.
The drug may be very quickly excreted or metabolised.
THe dosage may not be high enough?
I hope these ideas help! Good luck