There's a lot of discussion around how/ whether casters outpace martial classes in combat, but in my game I've actually noticed the gap more in investigation, social, and stealth encounters. Some bullet points to support this:
Investigation, Perception, Insight, History, Knowledge, Religion, and Arcana are all Int or Wisdom based skills. Monks may be good at the wisdom skills, but aside from that no martial class has any reason to invest in these skills. The same applies to Cha-based skills. I try to let players roll others skills as knowledge skills in the proper context (rolling Athletics to tell if someone is bluffing about his combat ability, for example) but mental stats are still overrepresented.
Most class abilities (for all classes, but especially martial classes) only come into play in combat. Those that don't ( like tongue of the sun and moon) are highly situational. However, there are many spells that offer massive benefits outside of combat. Martial classes only have access to class abilities and feats, not spells, limiting their specific non-combat utility.
An example from my game: My 6th-level party wanted to rescue some captive children from a hobgoblin camp. The Wizard cast Feign Death on herself so the party could exchange her corpse for some captives. The Cleric used Pass Without Trace, Blessing of the Trickster, and Disguise Self to get herself and the Open Hand Monk inside the camp. This left the Battle master Fighter with 9 Charisma to negotiate the exchange of the Wizard's body. Thus the Monk and the Fighter contributed significantly less to the success of the plan, and the Fighter was actively doing something she wasn't good at.
Now, my players are all having fun, and they're pretty good about trying to pass the spotlight around. But I can't help but wonder if there's a way to either:
Allow the Fighter and Monk to use their unique abilities more actively in non-combat encounters (like maybe there are skill or ability uses I'm missing?)
Better design non-combat encounters to offer clear roles for these characters to excel in
Encourage my players to make different plans that rely less on casters and more on the martial party members.
If you've encountered similar issues in your games, how have you addressed them? I'd prefer solutions that don't involve encouraging the martials to pursue a certain build, since I think they're already playing the characters they want to play. Giving them magic items is on the table, but not ideal.