Headphones

I've read a lot about fitting. Many say the vacuum effect indicates a proper seal. But I can never get that when I'm using spinfit cp155. Their surface is a black opaque matte finish and not grippy at all. They are a bit longer than usual ones thus a deeper insertion. It feels different because of the depth but I'm not sure if it's a proper seal. I can hear bass but I can't justify if it's what it's "supposed to be". (I mean I compared with other tips but they are all subtly different so there is no "correct" reference.) I felt like a decent seal but everytime I remove them, they kinda slide out very very easily. But they've never fallen out by themselves which is weird considering how loose they are. Unlike cp145 or other tips I've used, which are more grippy and take a bit more efforts to remove. (Xelastec and foam tips are extremely hard to remove in comparison. But that's the other end of the spectrum.)

So judging by my description, am I getting a good seal here? Or should I always look for a vacuum effect for a good seal?