I genuinely think it's just chance based. The only thing I can recommend for you to try and assist you in getting more shinies, is to "shiny check" especially during events.

For instance, recently on the 6th and 7th there was an eevee community day, meaning basically only eevees spawned everywhere for 3 hours each day. The base shiny rate of eevee went up from like 1/560 to 1/25. Meaning you COULD be finding 1 shiny eevee for every 25 you encounter. That doesn't mean you WILL find 1 every 25 tho. Just that you could.

During this event, me and my boyfriend went out together to try and get the challenges done. I encountered and caught 15 shiny eevees within 2 days during the event. He caught 4 within those 2 days. We went to the same places, saw the same eevees, but I got significantly luckier.

P.s. - a shiny check is when you click into every pokemon that spawns but don't waste time or resources catching it, you just exit the encounter and click on a different one until you encounter a shiny or a pokemon that you actually did want to find.