Probably the type of plant that's infested also.
Everyone's issues may be different in the end idk.
I can only speak for what worked for me and how hard of a time I had getting them gone.
I tried neem oil at first and it did kill everything you could see but they did come back after awhile even though the plant was isolated.
I switched to systemic and it took nearly 4 treatments spread apart when I noticed them being very small.
That was the ticket and how I ridded myself of them in the end.
I'm much more careful now. I don't take chances anymore.