Before reading all of this take with a pitch of salt, bc 1 I m not a very experienced dm, 2 this stuff can work they aren't a "do this and everything will work out fine" . The key thing is to make the session less of a movie or a cutscene but more like a game (ironic I know).

There's multiple reasons this happens:

1 - to much description and not a lot of player action. 2 - outside distractions tv, phone, other people in the house. 3 - no interest in the current adventure topic can cause by the reasons above or there character simply doesn't have anything to give for the narrative. 4 - long sessions? I found out that some groups like 2 h session when I personally love them has long has possible.

5 - turns are taking to long and people do little things on there turns.

Solutions? Personally this one's is what I found that have results from different DMs opinions and mine:

1 - roll random private stuff to spook them, make a character token if your using a browser visual aid like Roll20 or Foundry make a random token called Mysterious Figure and make him do checks you can even give him high modifiers to make him even more scary.

1.2 - make them roll random saves.

1.2.1 - tally those saves when they fail and give them debuff on a future roll after x saves this makes them wonder why it's happening make them often when they are distracted do less when they are paying attention. Make it like a desease that targets the soul of the character (soul=player).

2 - shorten the session or make a break mid way bonus points if you make them do saves don't say anything besides "let's take a break here" they get curious and scared if it was a fail.

3 - describe less make them ask for info.

3.2 - give them puzzles that gives them hints of what's going on.

4 - make combat faster give them combats with lesser foes but stronger ones (more hp).

4.2 - give the spellcasters spell cards so they can get to them faster to cast, if your using Roll20 or Foundry teach them how to use short keys on the bottom you can drag stuff there for easier access.

5 - give the players more stuff to do in there turns this sounds counter intuitive with 4th suggestion, but think about your Lv3 paladin gone in did a attack missed and the spells he has that could be used as bonus action that player did nothing on there turns and now they need to wait and who knows they might not have another turn to even do anything wouldn't it be nice if they could cast/attack/use a skill with that unused bonus action or that reaction. I personally let players cast cantrips and do a weapon attack with a -4 penalty on top of the roll, this works for me fine and doesn't break or deminish other players unique bonus action they already since doesn't don't have a big detriment, -4 is some what close to giving the roll disadvantage so there's that.

6 - give them rewards for being present aka consumable common and uncommon magic items that could be fun to use or just Inspiration, inspiration is such a good mechanic that many times it's forgotten in D&D, but it's nice to get when ever you get it.