Plot is not just a series of events. They are events that lead to consequences that leads to other events, based on realistic reactions from the characters toward conflicts.
Strong characters drive strong plot because of their strong reactions to strong conflicts that lead to strong consequences.
When someone says your plot is weak, it can mean a few things:
a) cliches -- predictable consequences from predictable characters doing predictable things. Unimaginative: rich girl meets poor boy, their families object, they run away.
b) the stakes are not high enough -- the conflicts and consequences are of the "ho hum, who cares?" variety: the character is bored and does something and it gets him a B+ in class instead of an A, when he doesn't even care about that class anyway...
c) weak characters and weak motivation/reaction -- the wants and desires of the characters are murky, undefined, or just weak: a man steals a shirt because he thinks he looks good in it.
When you have only one of the above, it's easier to recover or fix. But if you have all three, you're in a bind. Your plot and your story will feel flat and uninteresting.