"Tell me about a situation where you had to solve a difficult problem."
Do you all keep track of the difficult problems/projects you've worked on at work? How? Do you write them down? Do you just remember them when necessary?
That being said, what I'd do if I were in your shoes is create 2 copies of your resume: a copy for handing out and another copy annotated with notes for each job you list about what you'd want to highlight during an interview. Every time your current employer has a performance review, update both since that's when you'll be focused on your important accomplishments over the past review period.
I think my difficulty remembering the "difficult problems" stems from the problem seeming difficult, but after many flow states and eurekas, I end up with an accomplished task. Then it's on to the next task/feature/product etc.
I can articulate the problem but I can't articulate the eurekas and how I dealt with problem well