Are you seeing a faint white circle on the center of your plastic KAM snap, like this?
The white you are seeing is the natural mark that occurs when plastic is stretched / stressed. This article explains why if you want to read more about it.
If you're seeing this white mark, you're probably using snap pliers. The tip of the shank that flattens the prong applies very direct pressure which can result in the plastic cap being slightly stretched at that point. This is purely cosmetic and should not affect function in any way.
It can happen with any color, but will be most visible on darker colors. Pale/light colors will better blend with the white so the white isn't as obvious.
The white may get more obvious when more pressure is applied just because that means the plastic is getting more stretched. By pressure, I don't just mean the amount of physical force you're applying with your hand. Sometimes other things come into play to result in a certain amount of pressure, such as the density/compression/thickness of your material or the length of the snap prong.
Again, you will tend to see this happening more with pliers because of the pointy shank. The 3-piece professional-grade dies for the table press don't have that pointy shank, so you won't see that happening so much there. (The pointy shank is necessary to accommodate both the female & male sides, making it so you don't have to change parts to do one or the other; that's why the pro-dies for the table press have separate pieces that need to be changed out for each part.)
(I intentionally tried to break this snap so the white at the stress points is more clearly visible, so you can see what happens in an extreme case.)
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