This was an important lesson for me when I applied to grad school. First, I received a glowing recommendation letter from my undergraduate college adviser, who was also an instructor in three of my classes. But then I tried and failed to find a few more instructors to write recommendations. I assumed at the time that if I had received As in their classes that I ranked high enough for a letter, and I was initially shocked when they said no to my requests for letters, one after the other.

Now, twenty years later, I can understand completely that one semester with a shy undergrad who doesn't take the time to know you outside of class is not enough interaction for an honest recommendation letter. And how could they possibly have time to write letters for every A student who asks.

What did I do instead? I asked for letters from managers at my place of employment, which felt like a plan B but actually made for excellent recommendations. These were people I worked with 40 hours a week, who knew my personality and my work ethic. Even if my role there was unrelated to my planned course of study, they were more equipped to write those letters, and they did so willingly.

I like your approach to this situation, and I'll add that giving the students tips on who to turn to for letters instead might be another learning opportunity for them.