The two common reasons for not being able to smell perfumes are:
Firstly, perfume 'fatigue' - from wearing the same perfume all the time. TBH the usual symptom of this is the wearer using more and more until all around them have watering eyes... Simple solution is to have a few perfumes and rotate them.
The second reason is far more interesting and is probably what is going on with OP. This is anosmia to certain musks. Musk molecules are relatively large, and not everyone can smell all of them. Perfumers typically put several different musks in a perfume so that this effect is minimised (perfumers also suffer from this anosmia btw), but some musk molecules can actually 'block' the scent receptors in the nose so that the other notes cannot be smelled either.
This is further complicated by the fact that some people can smell certain musks at certain times (in my direct experience this can be linked to the menstrual cycle, which can also affect the sense of smell in general anyway) and furthermore some people can smell certain musks for a short period of time before the blocking action occurs.
White musks and perfumes with a white musk base (which is quite a lot of perfumes!) are most guilty of this. SJP Lovely and the Narciso Rodriguez musks are famous examples of this - some people say they are like spraying water or that they are faint and delicate florals.
Funky, animalic musks can also do this - if you take a really dirty, crotch-like musk such as Serge Lutens Musc Kublai Khan, you will often get puzzled wearers who claim it smells as lovely and soft and gentle as a kitten's tummy (me, for example. My husband however says it smells like sex!).