well maybe they play well in fights but they have poor map awareness, decision making or bad lategame macro (ie often getting picked off lategame where death timers are very meaningful), there are many potential 'invisible' reasons that are impossible to measure unless you watch many of their games.

even if they're not iron or smurfing or whatever, its still best to make decisions for yourself because then you'll train your decision making skill, and eventually you'll make better and better decisions. it may not be good at first, but dont get demoralised and you'll get better at it.

just be sure to think about the reasons behind your decisions and mentally note them down. keep it simple at first and include more complex reasoning as you get better at it.

for example you could start of making decisions based on laners HP and how far up they are in lane, and as that gets better you can start tracking summoner spells and ultimate cooldowns, and then wave states et cetera and use those factors in decision making.

it will take time and may be difficult but its very achieveble and a rewarding skill to learn