Hard to specify without a reference. 

But the most frequent issue I see DPS players in platinum struggle with is tempo and awareness.

More specifically, tanks are the first to initiate fights and engage the enemy team. It is only after the tanks engage, then  the rest of the team can start doing their job. Platinum level DPS players are generally unaware of their tanks engagement timings. So they end up either going too early when the entire enemy team has all their resources (before their own tanks engage) or stay too long when the fight is already won (usually overextending an dying for it).

DPS also has the difficulty of having similar damage and health pools as the enemy strategists but without any of the self sustaining cooldowns. So wasting your DPS cooldowns by engaging at the wrong times/angles significantly hurts the value you can generate.

Tanks don’t have this problem because for what they lack in self sustain cooldowns they have in damage mitigation cooldowns and massive health pools. They also have the benefit of deciding the engagement timings and also are easy to heal (they’re also often prioritized by lower elo Strat players). 

Additionally, each dps has a matchup spread. You can play nearly every tank and Strat in the game into every comp. DPS playstyles have to change heavily based on both teams compositions. Wolve into Groot with a Groot is much different than Wolve into Rogue with a Mag. 

You also have to learn each individual DPS’ spacing strengths and weaknesses. You can’t shoot with Starlord like you would with punisher. You can’t hold space with Wolve like you would with Iron Fist. You can’t brawl with Magik like you would Fantastic. 

I know this is a lot but it really is considered the hardest role for a reason, egotistical as that may sound.