Feral Ghoul not attacking me anymore

[HaaS-19]

I had an objective to survive 15 feral ghoul attack and the reward is a lunchbox.
but now feral ghoul is not attacking me anymore so I can't progress to get the reward, right now I only getting attacked by the deathclaw.
Do I have to do something to trigger the feral ghoul to attack me?

[Humdrum Sebby]

It's pretty much random so you can only wait for them to eventually come. May be a better idea to just skip that objective.

[Carolynne]

This is something I have had much problem with as well but with raiders instead of feral ghouls.

The hostile encounters are triggered by milestones, based on how many dwellers are in the vault and how deep down you've built the vault.

Once you've reached a milestone and your vault is considered stronger, the first tier of hostile encounter fades and second tier is activated. The stronger you get, the less their previous tier encounter will occur.
This is a design flaw for anyone who wants to complete the objective or game achievements of hostile encounters.

As of now the only way to bring back a lower tier encounter is to create a new vault and remain weak until the challenge(achievement) is completed, but there's no solution for the objective rewards.

You are only presented with few options and they're all horrendous, you either have to wait or cancel the objective or play illegitimate.