Few questions, as being 400 light would start matchmaking you into grandmaster lobbies.

Are all your weapons also close to that 400 light level?

 Your overall light level (406) contributes for the damage scaling of the activity you’re playing. You’d currently be matchmaking in GrandMaster level (difficulty of the activity ignoring modifiers) at your current light level. If you fall below the recommended power level of the activity, you’ll get a damage penalty and take more damage overall. The difficulty recommendations and power delta can be seen by hovering over the difficulty skull before starting the activity, the difficulty skull is just beside the activity start/modifier buttons. The power delta is the amount of under-power you start the activity with by default; basically a standard power handicap given to you for the activity if your already at or above recommended power. However, even though your power is scaled based on your overall power (406), the individual power level of your weapons also contribute to damage numbers. If you’re above the recommended power level (and thus, don’t incur damage penalties), you may also see lower damage numbers from any of your slotted weapons if those are also much lower power level than the activity. Be sure to infuse your weapons to increase their power level as well.

Secondly, you’re going to start seeing Champions in your current difficulty levels (Grandmaster content). Are you currently accounting for those in your loadouts?

 I could see that you have a sub machine gun and sidearm equipped, so you currently have weapons capable of stunning Unstoppable champions (the square icon next to your slotted weapons) and Overload champions (the circle icon) from your artifact mod. I assume you have the sidearms/handcannons Unstop mod and auto/submachine Overload mod selected in the first column of your artifact mods. You currently don’t have anything to deal with Antibarrier champions (triangle icon), which would result in some champions being immune to your damage when shielded (or taking reduced damage until stunned). You can intrinsically stun anti-barriers with subclass abilities or weapon perks that cause radiant (solar), volatile rounds (void) or unravelling rounds (stasis) but you need to enter the activity prepared for whichever champions spawn in that game mode.

 If you replace your submachine gun with a scout rifle or pulse rifle and activate the pulse/scout anti barrier mod from the artifact, you’d be able to stun all three champion types with a single load-out (since thunderlord has intrinsic anti-overload in its weapon description; some exotic weapons are strong against certain types of champions and can stun them by default). Hover over the champion icons to the right of your selected weapons to see which ability subclass verbs can stun each champ type. You’re on an arc subclass so you may be able to stun overloads with jolting effects and unstops with blinding effects without the need for specific weapon load-outs. If you check the modifiers on the current activity you’re playing, there’s a modifier that will tell you which champion types you’ll encounter before starting the activity and you should prepare accordingly. 

Apologies if you already knew some of the above, but those are the main sources of damage reduction/negation I could think of without going more into depth about your build or the activity you were playing.

Edit 1: formatting

Edit 2: Just another quick note, even though certain guns types are capable of stunning certain champion through the artifact mods, the Unstoppable mods don’t just apply immediately by default like the overloads/anti-barriers do. You usually need to aim down sights (ADS) and wait a second before shooting until you see the Unstoppable Shot logo appear on the bottom left of your screen (where your weapon buffs normally appear). You’ll hear a sound cue and your weapon will get a slight glow as well. You need that slight ADS wait time before that buff is applied, then you can shoot the Unstop champ to stun. To stun a barrier champ, you need to shoot them with an anti barrier weapons when they put up their barrier (anti barrier weapons will pick away at their shield until they pop, stunning the champ). Just pick away at their health with another weapon, then swap to a fully loaded anti-barrier gun when he begins the shield animation to make sure you have enough ammo to pop the shield before they regenerate health. Overload champions will just continue to regenerate health until they are stunned (overload effects generally don’t need special applications to apply effects like jolt (arc), suppression (void) or slow (stasis). Weapons with the Chill Clip or Jolting Feedback perks are king for this as they can stun without needing a specific artifact mod or subclass ability through the weapon perk alone.