Using a baking rack might help (as suggested in this link) and, of course, not holding back on the butter or olive oil, might well help:
https://www.wholesomeyum.com/crispy-baked-chicken-legs-drumsticks-recipe/
(As that's a US site, it uses Fahrenheit temperatures. To save you googling it, 425 F = 220 C, which is Gas Mark 8)