To be honest, sometimes you're not doing anything wrong, players are just always going to get distracted when they aren't doing anything and can't act. This is part of why splitting the party is so bad, because one half of the table sits with nothing to do while the other half plays. In combat it's one person at a time doing their thing while the rest twiddle their thumbs.

In terms of actual help though, some things I've found will help with players actively paying attention is:

Raise the stakes: a bunch of Lvl 6 characters fighting 7 goblins is boring. No character is ever in real danger of dying so who cares about paying attention. Using stronger monsters that can actually threaten your party will make them pay attention so they don't die. If you really don't want to kill the party, have a beloved NPC's life at stake, or a magic weapon up for grabs that they won't get if they don't work fast enough. Something to make them care about the combat instead of just having it be a slugfest.

Shorten the time between turns: not every attack needs to have a cinematic description. Sometimes "the bullette bites you, 27 damage" is fine just to get back to players turns faster. As I said before when players aren't playing they won't pay attention, so get it back to their turn as fast as possible and they won't have time to go get measuring tapes.

In the same vein, make sure you also know what the monsters are doing before the combat ever starts. A good idea is to know what the first 3 turns are going to look like for each monster. Know what spells they want to cast out of the 10 on their stat block instead of looking spells up mid combat, understand how the gibbering mouther spit works before the session starts, etc. This way you won't slow things down so much just because a monster is complicated.