How couldn't the Pred see Arnie?

[Aeus]

Filtering comes from simple observation.

Real heat vision is what we see when the Predator removes its mask. Before then the filmmakers had to tamper with the environment to get contrast.

Always thought this was the case.

[Predator Queen]

mud is cold therefore it made Arnie look cold so the heat vision couldent see past arnies cold covering

[ShadowPred]

Filtering comes from simple observation.

Real heat vision is what we see when the Predator removes its mask. Before then the filmmakers had to tamper with the environment to get contrast.

Always though this was the case.

Heat vision is blak and white.

[Xenomorphine]

That above link with the synopsis pointed out that mud did mask heat for a while, but that the heat started to show after a certain length of time. That explains the river scene. Not sure about the others, but perhaps the character took a fresh supply in pouches to keep coating himself with.

[Gill Predator]

mud is cold therefore it made Arnie look cold so the heat vision couldent see past arnies cold covering

Yes he is cool at that time