How can I let my players catch Droki from Out of the Abyss?

Question

In Chapter 4: Gracklstugh of Out of the Abyss, my players are supposed to catch Droki. But they just can't do it.

The party is comprised of a Drow Ranger, a Firbolg Druid, a Dwarf Paladin and an Aasimar Warlock, all level 4. They're currently in the Whorlstone Tunnels where they failed catching Droki once after having twice failed in the city.

From what I understood, Droki is supposed to be roleplayed a bit like the White Rabbit of Alice's Adventures in Wonderland. According to that, I make him flee upon feeling a danger. The fact that Droki has boots of speed¹ is the main issue here as he keeps running when the (noisy) adventurers encounter him. I don't want to hand Droki over to the party, but they don't even try to get him anymore. They seem to think that I am making it impossible for them to catch him. The one time they had a half-decent idea (bombarding him with arrows and spells), they had a sequence of bad rolls.

At this point, all I can think of is providing them with one or two potions of speed and specific spells as loot. But maybe there are other ways which I haven't thought about. This has been a challenge for me for two sessions already and I'd really like them to catch the Derro in the next session.

What needs to change?

I'm ready to change the way I handle Droki in order to do so. I have already thought about something like "oops, Droki used the whole of 10 minutes of speed" which has my best favor currently, but as I said I don't want to simply hand him over.

How can I let them catch the NPC without it being a giveaway?

Even further to be helpful to further DMs, how should the Droki situation be handled in Out of the Abyss, normally?

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¹ double walking speed, opportunity attack rolls on the wearer have disadvantage

Answer 1

Without nudging your players into particular actions, there are many ways you can set Droki up so the characters can have the best chance to catch him. I will enumerate them broadly and leave you to the idea-generation:

Droki is unattentive in the Whorlstone tunnels

Inside the Whorlstone Tunnels, however, the Derro is distracted by ever-present threats, giving the party the best chance to catch him.
[...]
Droki's route takes him all around the Whorlstone Tunnels, and he's too scatterbrained to notice anyone following him. (Out of the Abyss, p. 68 & 69)

The characters could catch Droki by surprise while they follow him around the tunnels.

Use Random Encounters

The Whorlstone Tunnels aren't empty, and Droki is as likely to find random encounters than the players. Think up a scenario where Droki is interacting with one of the Random Encounters in the dungeon where the players can get the opportunity to snatch him up. What I personally used was:

The random encounter with the Flumph. The Flumph knows much about the cultist and their activities, so it's only natural that it knows about Droki as well. I let the Flumph tell the PCs that Droki follows a specific route and hardly deviates from it. The Flumph can then point them to where he will go next and the PCs can setup an ambush.

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Lastly, capturing Droki is optional, their main goal is to:

... follow Droki, see what he does and where he goes, and report back. Or if they see an opportunity, seize him and either bring him in for questioning, or kill him and bring back evidence of his activities. (Out of the Abyss, p. 63 "Search for Droki")

If your players have lost interest in actually catching him, don't force them into it. Give them the evidence they need to bring back to Errde (when they earn it, of course) and that's that.