So over the last week or so I've had a Heracross at the front of my party searching for a shiny Heracross on Route 33 (east of Azalea town) and when I headbutt the tree I was getting a 100% drop rate when i headbutt the tree, once I got the shiny Heracross I swapped out my Heracross for a Fearow and try to located a shiny Spearow but the drop rate is more like 50% now, so not really sure what changed, I am using the same tree as before as well.
Is there something that can decrease or increase drop rates when headbutting? I've tried looking on google and it seems to suggest in each area trees either drop pokemon or they don't, couldn't find anything about the rate if encounter.
Pic of my newest party member too 😊
UPDATE: Thanks to redditor KSBmite with the Keen Eye pickup, the Fearow I was using had Keen Eye and it was reducing the encounter rate.