The explanation for the 'not-being-seen' phenomenon I have found is this -
Everything is alive! Everything has consciousness to a degree! That includes thoughts, emotions and believes! Energies, especially negative ones (anger, fear, guilt, arrogance, disconnectedness, etc, etc) need to be felt, and believed as real, to keep on growing and multiplying. Most empaths are 'lighter' than the average human. More unconditionally loving, non-judging, feel more connected, etc.... They feed negative energies less. If anything we can help enlighten and dissolve negativity. So we are a threat to negative energies! Ergo a person who still has a fair amount of negative energies in his/ her being - those energies will do what they can, so that that person forgets about us, does not see us etc, etc....
But I also belief that every time we do manage to make contact, think lovingly of someone, or perhaps are even just around more confused beings, we plant a seed of Light, and those seeds will come to bloom. So hang in there all! Let's not feed the 'loneliness energies' .
If i feel lonely, I try to shift my focus from the physical human plain to the spiritual helper plain. Sure, I cannot always see or feel them, but over the years I have become confident that there is so, so many. We might all have a million or more friends up there (or below us (Earth energies are very helpful too!). Or if you need to hold something physical, I get comfort form holding my crystals/ rocks during meditation, which I connect to! I am not allowed pets where I rent, but supposedly pets can feel much more connected and loyal than many humans too!
And yes, from the outside my life might look lonely, but it has become more sociable again over the years (took 10+ years, but then I did not have forums like during my darkest years ). And I rarely feel lonely nowadays. And as mentioned by chrisis - perhaps start looking in other places for friends. Places were people are more awake already - meditation classes/ meetups, yoga, shamanism, etc etc. There is a higher likelihood of connecting more there. Just do not go in with too high expectations, it could be that your Light will have to shine there for a while too, before acknowledged. I have started attending meet ups from last year, met some nice empaths. Have attended more spiritual events again too, but they are still hit and miss, and no besties yet )