I've been wondering about this a lot as I have been in both sides and I have my own psychic "theories" why this happens, but I'm still not sure.
-Legit psychics form consciously or unconsciously a cord between you and them so they can draw info from there. If something isn't right and your energy isn't compatible with theirs, the connection will be off. Things that may cause a poor connection of this sort: psychic was in a bad mood or drained following a reading or readings for other people, you don't want to subconsciously hear that psychic's answer/the truth etc or you may simply be very very different people with different auras, different astrological compatibility, and just different energies around you. It's like a temporary marriage and cord-tie connection. If you aren't compatible, it's just not going to work.
-The spirits of these psychics don't like "some" of us. Again assuming that the psychic genuinely consults spirits for answers, these spirits for good or for worse do not like and help everyone that asks them through this psychic. They may be human discarnates or trickster spirits but they have their own personalities as well and if they don't like you, they will spin entire tales, lies, and stories that aren't true. If, however, they do like your energy and aura, they may be more helpful, honest, and accurate.
-Regarding the spirit thing again. Not even GOD or Satan channeled through a psychic would work for everyone and there are two reasons why: a lot gets lost in translation and second, if such thing happened and the psychic was 100% accurate, they would be deluded into thinking they were all powerful and omniscient beings without any motivation to work on their skills. Sometimes their guides and masters fool them on purpose so they keep growing as readers.
Also, some psychics do see the truth e.g let's say that your POI never loved you but choose not to tell one client about it because they sense that the client will get depressed or even have suicidal thoughts because of their insight.
However, in most cases, I believe the psychic is just energetically drained and has mixed up energies from past client readings after failing to take a break in between reading sessions. This is why some reputable psychics state that they don't encourage physic addiction and having too many readings about the same thing over a short period of time.
Curious to know what other "theories" members have about this situation as well.