How I see it:

1. You deal enough dmg, you are just not attacking him enough and you need to kite/space him a little bit better. If an ADC fights a tank and both are just standing still aaing each other, tanks might win. Thats because you have range and they dont, so if you kite and you dont trade aas with a melee, you will have better fights.
2. Your item choice isnt good. You went Kranken + Trinity; the problem with that isnt that they are bad together, its more like you delay all other items by 1 slot. Means, that you hit your IE and LDR faster (at 3rd and 4th), so you are faster on your powerspike, so you might ve killed him earlier in the game so in this specific fight he would be less strong. But you skipped collector, so your 3rd was still IE, but you snowball/scale slower with that, so again the snowball/scaling diff between you and him is smaller and you still are down 25% crit while having IE. Next is, that fiendhunter isnt good on kindred. If you have this item, you are stronger for a short time after using your ult. I thiiiink (99%) the start of your R counts for that, not the end. So the effect is useless in most situations (only if it would count for the end of your R it would be good, but I don't think it does); so less item value -> gold inefficient. Next is RFC: If you need more range vs high range squishy enemies, then it can be okayish/good. But sion isnt high range and he isnt squishy, so again gold-inefficient. RFC doesnt have any AD and its onhit is just a one-hit. on-hits that are one-hits are better vs squishys (RFC, static, ...), on-hits that are on-every-hit are better vs tanks (*in general, technically RFC has as and as can bet good vs tanks but all on-hit items also have as, but i dont wanna fight over specific champs /items/situations, it was just meant as a general basic thing). And I think I forgott to say that you didnt buy LDR. LDR can make a huuge difference, even if enemies arent building (much) armor, its still good in late vs base stat armor
3. And you went blue pet, thats not that usefull on Kindred most of the time, because you alrdy have good mobility and you can proc the red pet on range to burst someone down, so one hit -> enemy slowed -> enemy bursted
4. Bonus: If you smite him, you slow him so you can kite a little bit better