Multi check are too easy?

When asking for a skill check that all the players can attempt, it is quite easy to have at least one player to succeed.

Indeed, a 15 DC investigation (that is a 30% chance with a +0) is quite easy for a group of 5 PC (83% chance to succeed). Even a 20 DC would not be that difficult if there is enough people with a +3/+4.

How to manage group check to avoid the check to be too easy ?

To notice that I am not talking about a group check (in that case it looks that half+ of the player need to succeed), but rather a check that everyone can attempt one at a time (opening a safe, decipher a scroll, looking at an ancient item to know its history…)