In "Reading Women Correctly," Robert Lindsay discusses the kind of flirtatious signals women give when they are interested in a man. At the end he notes how some commenters complain about not getting any signals at all. These guys, known as involuntary celibates or incels, are basically invisible to women sexually.
Robert describes instances where women are almost literally attracted to him in a carnal sense- in one case, a girl was staring at his chest hair as if it were a prime rib. This type of animal sexual attraction is rarely experienced by incels, and is a huge stumbling block for them because it's very difficult to flirt with to a girl when she isn't putting out any signals of interest to begin with.
Furthermore, this lack of interest prevents an incel from fine-tuning his attraction radar, so he can better piece apart when a girl is flirting or not. This causes the incel to stumble badly whenever an opportunity arises, if it ever does. In the best case, the incel ends up broadcasting his sexual inexperience, and the consequences can be far more severe. These types of outcomes only reinforce the incel's sense of inadequacy, as he is already suffering from a lack of affection and validation to begin with.
I consider this phenomenon to be the core problem of incel. It is a vicious, self-reinforcing circle, that only gets harder to break out of with time. Poor attractiveness (for whatever reason) leads to a poor reading of signals, if any are sent by girls to begin with, which often results in harmful psychological repercussions.
I will discuss ways to break this vicious circle in a future post.