Yeah it's a known problem, discussed in this topic as well - https://forum.plarium.com/mech-arena/702_game-discussion/1757785_is-the-game-facing-a-problem-registering-assists/ .

Mods are a big reason for it. Here's what I had observed, and the behaviour is the same today: If an enemy mech has mods, and if I fire at the mech, destroy the mods and then continue firing at and damaging the mech, then if somebody else kills it I don't get the assist. But if I start firing at a mech whose mods are already gone and somebody else kills it, then I get the assist.

"ShieldArena"

I like the nickname. It's exactly that (sort of). Mods should be taken out (which will never happen). It should have never been introduced. It would have been better to just rebalanced released mechs/weapons while making adjustments to new mech/weapons releases. They gave themselves more work trying to balance and rebalance around mods now with the current and newly released items. Some items became almost useless (i.e. Railguns for example) due to this. But, the mods do give another paying layer for Plarium which, I guess, is good?  

It's true that mods made several old metas (weapons which were released before the introduction of mods) useless. I won't completely oppose introduction of mods (since it helps low-HP mechs to work well) and increase their survivability) But Plarium should've buffed these old weapons so that it could compete against modded mechs. The devs shouldn't have let old weapons rot in the inventory, and buff these weapons appropriately (like giving them passive effects or Shield Piercing) 

I think mods are a good thing, otherwise most weapons could kill you with just one round, 

But saying that, we definitely need better ways to earn mod parts, it takes forever to max them out

Players still gauge what weapons to use to be able to finish an opponent with one magazine. Even content creators judge what weapons are good by this (take the Burrow Beams pre-nerf for example). With the introduction of mods, Plarium is now having to release mechs and weapons to counter the mods. They should have just re-balanced older mechs and weapons to current specs to keep up with the newer products being released. If they have just done it this way, weapons like Railguns, EM Rifles, Graviton Beams, etc. would still be viable for gameplay.

For how long do you play this game? I played it before mods and game was fine. Since MODs game is continualy declining. This game is only for P2W with thick vallet. 

Two years and they did nothing? This game definately hates their players. May be it was mistake come back to this game again. Even moderators gave up on this forum.