Im having a problem with understanding the invisibility spell.

I've read everywhere that it's still possible to be detected when under invisibility because NPCs will hear you moving around. I've also read that the way to remedy this is by either casting muffle or wearing gear that does the same (namely: the Nightingale boots). When I cast invisibility, I put on the boots and NPCs are still able to detect me like I'm walking around playing the trumpet. I've tried casting muffle as an extra precaution; Nothing. Even sneaking I have the same problem. I'm not wearing any armor at all and I'm walking on solid ground without anything being kicked around... Why does the invisibility spell not work at all??

Edit: I forgot to mention: I'm not brushing against them, casting spells, hitting them or otherwise interacting with anything. I'm slowly walking.

are you casting invisibility after they have already seen you?

I've said it before and I'll say it again, invisibility in Skyrim isn't really invisibility. Once you are seen, invisibility will not work after that. The enemies can magically find you whether you use muffle or not.

Lol... No, of course not. I'm casting it well beyond their visibility threshold. And no, it hasn't run out by the time I get to the enemies (~15 seconds left). Do I have to be sneaking for it to work? I have tried that with the same outcome.

I'm starting to believe this "they just magically know where you are" theory.

Muffle doesn't make you completely silent. Not those boots and not even the muffle spell. They reduce noice, not completely block it out. Walking generates much more noise than sneaking, which is why they hear you. It's silent enough for them to hear such noise.

Once you've somehow initiated combat with them, they will always know where you are so long as said combat goes on because they too have a HUD with a red dot.

Okay well that kind of makes a bit of sense. So in this case, are the Nightingale boots an equivalent replacement for muffle? Or are they a higher grade of muffling? Basically I want to know if I should just sell them and cast muffle instead.

That much I did know. Maybe not the red dot

Guess I better level up my sneak. I just want to Mass-Paralysis/Mayhem everything lol.

Do any of these spells do what they're supposed to?