It will sound juvenile, but have you tried writing fan fiction? The setting, characters and relationships are already in place, leaving you to worry only about a plot and how to move it forward. Fan fiction is a great exercise for writers who struggle with plot because it's the only aspect of writing they actually have to think about, considering everything else has been created for them.
Other than fan fiction (if you've tried it and it doesn't help, or you have some issue with it) I would suggest an A/B character exercise in which you have person A and person B and neither could be blander or more under developed. But that's OK because it's not about those characters. It's about the conflict between them, or about something that happens to them. Start generic, then narrow it down, make it more specific. This is adapted from an acting exercise.
If you want an example of an A/B sketch, just ask.
Another exercise I could think of is to take something mundane and every day and turn it into an adventure. This is more an exercise in descriptive/figuritive language and voice, but I think it can help with plot as well because you'll be thinking about how to make something boring interesting. For example, a kid is going to the store to buy milk for his mother. But maybe it's the first time he's allowed to go out alone. How would he see this? What might he turn it into, in his imagination?
Lastly, drop yourself in a high-stakes scene or situation. One of our prompts for writer's group years ago, when we had one, was "The sun was hot, the ocean was vast, and the sharks were beginning to circle." [
link]
Right there you have a high-stakes situation. How are you going to get your character(s) out of it? You don't need to do that sentence. There are a million other prompts that'll do the job.
Anyway hope this was helpful a little.
