First, talk to the kids who are missing and try and get a feeling for what their home life/ life outside of school looks like. It sounds like you are already doing this which is great.
I have 1 kid who consistently misses because of random family things coming up and another because he lives 45+ min in traffic cause his family moved and he is now a transfer to our district.
With these kids I try to be very patient but I also remind them that they can't get better if they aren't here. Normally both of these kids are strong players and they can make up the time in class.
IF you can rehearse outside during class, that needs to be where you do most of your work gets done.
I had a good chunk of kids missing as well yesterday and the rehearsal was frustrating because we were supposed to learn our ballad but it was hard with so many gone. Today we went outside during class and filled in the missing spots and got it done. The kids who were gone made up for the time and the kids who were there yesterday were able to clean what they just learned.
Don't stop the after school rehearsals, and rehearse the kids who show up, the ones who are skipping will either 1. Make up the missed time because they want the band to be better, or 2. they likely won't do band because they'll realize they don't want to put in the time or effort.
Losing kids cause they don't want to put in the work is a good thing, losing kids cause they feel like they can't do everything is a bad thing. Be patient and understanding