The general rule in psychiatry is that if they failed they never wanted to die in the first place.
I'm not clinically licensed, but my understanding is that while this is a common popular interpretation, it is not part of any diagnostic or treatment criteria. Often times, the exact opposite is stressed, with patients/loved ones encouraged to seek intervention following any attempt, regardless of the method or its actual lethality. A psychologist may
think you're full of shit for trying to kill yourself with a handful of aspirin, but he's required to take it as seriously as the guy who put a handgun into his mouth and lucked out when it jammed (lest he get fucked in the ass by a lawsuit/lose his license.)
Most people who attempt suicide only do it once. Some of those people make attempts solely for attention, but sometimes that desire for attention is because they feel that they simply have no solution available and previous attempts to seek help failed. If caught in a bear trap, choosing to gnaw off your leg for either escape or to bleed out are both preferable than starving to death.
But also, if you've attempted suicide once, your chance to die by a later attempt increases dramatically. It's not always that people survive because
they secretly want to live, and others don't, and once they fail then it's over and they move on. Some people are just ignorant of the difficulty in actually reaching lethality with their attempt, thanks to things like your body's survival instincts fighting against you. You can read the PPH front to back as much as you like, but your baser brain bits are still going to do everything they can to get you to stop when you put the noose around your neck or the barrel against your head.
Given the impulsive nature behind most suicide attempts, many are likely made without the preparations and information necessary to succeed. For every one that goes and checks online on what's the best knot and rope to use, there's likely ten that just say "fuck it" and down whatever they find in their medicine cabinet, hoping for the best. Personally, I've felt that it would be beneficial for research and treatment to separate suicide into something akin to an acute/chronic analog, based on the impulsivity of the act and time between ideation and action, but I'm not familiar with any research supporting the distinction overall. Some studies look at this, but some just pull from sources where the only information is that someone died by suicide, along with the method. Unfortunately, trying to piece together how long they planned it after the fact can sometimes be impossible.