Fixatives "glue" the volatile substances in a lure together by lowering or equalizing their vapor prassure.
Good fixatives have very low vapor pressure of their own, meaning molecules pop off the surface of the mix very reluctantly. Substances with high vapor pressure are eager to evaporate (think acetone). Fixatives literally trap substances with higher vapor pressure, and they are released slower and more steadily. A good rule of thumb is, the bigger the molecule, the lower its vapor pressure. This is why musks are good fixatives - they are big, long and usually have big carbon rings in their structure, so they act as a "velcro" for smaller more volatile molecules (such as minty- or fruity-smelling esters).
One of the effects is that when you smell a mix blended with fixatives, your nose is not overpowered by the sharp volatile notes and you get a chance to smell the heavier notes. This is mostly how the perceived "amplification" effect works.
Our noses are not just less sensitive than animal noses, they are untrained because we don't really use olfaction much compared to animals. Very often we smell a mix too closely and it completely clogs our receptors before we get a chance to understand any nuances. It's very possible to practice and learn to sense subtle smells, but it takes time.
There are other factors at play such as detection threshold, it often has little to do with volatility and vapor pressure. Some substances that are important for a species, for one reason or another (such as pheromones, or substances signalling contamination, or toxins, or presence of water, or calorie-rich foods) can be sensed at very, very low concentrations. Mercaptans in skunk spray or some burnt smell substances are good examples of low detection threshold.
