I have this issue in "normal" years, but this year with so many of my kids doing online school it's even worse.
How do you get kids to listen...to anything? I have kids who will turn on a metronome, then proceed to play whatever tempo they feel like (and often a different one every measure). They have no concept (or maybe are just afraid to answer) of whether they are the melody or the harmony. When we finish a rep, they can't identify who had the melody (even if they WERE the melody!). I give online kids an assignment to play along with a recording, and they'll have the recording playing, but they don't play along with it. I just watched a kid completely ignore that he had an 8 measure rest to begin the piece, then looked very confused when he reached the end and the recording was still going.
Obviously I have not taught this skill well. What am I missing?
Treat everything as a skill that you must train them to perform. So before they even have horns in their hands, and you are teaching rhythm (foot tap included of course), ask them to listen for the "together" of the foot tap. Don't forget to reinforce that every time foot tap is audible.
When counting, concentrate on the right syllables AND the precision of their counting. (Precision can only be judged by listening.) Train them to listen for precise counting.
When they play that first note, encourage them to listen to each other and start it together at your downbeat. If it is not good (it won't be), let them hold that first note as you normally would. Make whatever suggestions you need to make for tone etc., then start them again and ask for a better (more together) start.
I imagine you get the point.
As you work through their development, "listen" must be a recurring encouragement so that they have that as a skill that they use daily.
Of course, you find different ways to approach that, such as:
Was that together?
Who started before the rest of the band?
How do we know if those eighth notes are together?
Etc.
Keep asking good questions of people who know more than you.
Break things down into the smallest piece possible and ask questions about things. Ask the students that never answer.
Sometimes it's an issue with counting rests on the time signature. If they have eight measures of rest in 3/4 or 2/4 it's much different than 4/4. Explain everything in bite-size pieces. A simple concept that most beginners and some middle school students do not understand is that whole rests are the value of the measure in the current time signature. It could be 4 beats, 3 beats, or 2 beats.
For tempo issues with a metronome, break it down to quarter notes for one measure, then two, then four. Put eighth notes into that rhythm next with one measure, then two, then four.
We use rhythm bee and when I went from kids counting to playing the rhythm there was a huge realization of how many students can hide when counting. After understanding how to count with a met the rhythms we then play one of their first notes on that rhythm. It's no wonder they were struggling reading music when they can't play in time on a basic rhythm. It has improved considerably in the last few weeks just changing over to playing the rhythms. Sometimes students have disconnect because there is more going on when playing than just counting. I do a lot of hissing as well for articulations to clean them up.
Always ask questions such as 'Susie, who has the melody at measure 64?' Ask two or three students then you should explain what a melody is and why it's important to know where it is in the music.
I'm guilty of throwing too many things at students at once myself. Sure they can get through things but it's only 75-80% correct. Go back and work on those details/concepts. Many students have become professional in their ability to hide and not be heard/seen. Small groups to clean, small concepts to cover, and daily building. Don't move on to something new until they get those concepts you are working.
I've had the most success with this when I've been able to do pull outs and smaller sectionals because the kids feel a sense of urgency to stay in the moment when they know there isn't a crowd to blend into.
Likewise, if you have private teaching options available that individual work tends to go a long way toward kids learning how to stay mentally engaged.
Finally, just discussing mental engagement goes a long way for me. Reference zoning out in history class (they'll know exactly what you mean) and explain that staying mentally present is a learned skill they have to work at.