Suggest to check review of your lens

1). There is best aperature of each lens, for example, 200-500 best aperature is F5.6, if the aperature is small, the image quality will be reduced.

https://photographylife.com/reviews/nikon-200-500mm-f5-6e-vr/

2). You need to understand the deep of the field and how big it is at certain distance. Usually need to use the largest aperature in condition of enough deep of field

3). Avoid noisy(High contrast) background, since it will mislead of camera's focus system.

Sorry can only type english in office :-)