Are there any warning signs that a person might be suicidal? What are some examples of behaviors that were discussed in The Bridge documentary before people committed suicide? Why do you think family and friends might not have taken these behaviors seriously?

The thing to keep in mind with this documentary is that, like with all documentaries, there is information left out, and the documentary makers choose what to leave in very carefully. In addition to this, recollections from family and friends of people who have attempted or died by suicide are going to be understandably affected by their feelings: in the documentary we see grief, sadness, guilt, and anger, as well as relief and acceptance. The stories we tell about how people were before they die are always going to change: hindsight is 20/20.
The interesting thing about this documentary is that it presents the perspectives of the loved ones of so many different people. The warning signs and behavior leading up to them jumping from the bridge are mostly varied, but there are some common themes.
One example of behavior that was discussed is that multiple people had been receiving psychiatric treatment and had either suffered negative side effects or had not experienced any relief from symptoms—so they had ceased treatment.
In the case of Lisa, her family observed that she had been on and off medication for her schizophrenia over a period of many years. It seems like her family had taken this very seriously, but it had happened so many times before that it was reasonable for them to expect that she would begin treatment again or that she would be okay. Her mother and sister also hinted at there being issues of their own safety around Lisa.
In the case of Phillip, he told his father that the "medications made him worse." As well as this, while he was in hospital for a previous suicide attempt, he had said to his father, "My next attempt won't fail." His parents tell us that this upset them and that they had been expecting him to die by suicide but also that they felt powerless to help him.
In the case of David "Ruby" Rubinstein, he had been talking to a friend about getting treatment for depression, but he didn't have health insurance. He mentioned to his friend that he wanted to take medication but couldn't afford to, so she gave him some of her medication to take. David's situation is almost the reverse of that of Lisa and Phillip: he was desperate for treatment but was unable to get access to it. In this case, his friend mentions that she took his anguish, and the warning signs, very seriously. But she also took seriously the fact that he might feel ashamed or embarrassed and need some time to himself after he broke down crying in front of her.
What these three examples tell us is that it's not that the family and friends didn't take the behavior of their loved ones seriously; it's that they didn't know what action to take.

If this documentary or post has raised concerns for you or a loved one, please contact a health professional or your school's counsellor. You can also call or text somebody 24/7/365 through a counseling hotline, which I've linked to below for the United States, Canada and Australia. If you're in another country you can Google "lifeline" and contact somebody that way.
https://suicidepreventionlifeline.org/

https://www.crisistextline.ca/

https://www.lifeline.org.au/