I have found that if you use a zone, that you need to keep any sprite as close to it as possible. The top left seems to be the best. But if they get more than 4 tiles away, sometimes they do not work right. Obviously, this is not an issue with the physical contact switches because you have to come directly in contact with them. But possible countdown timers etc. that go with the physical contact switches may need this as well.

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Only the the objects/Sprites you can see on your DS-Screen + approximately 10 tiles in every directory gets loaded into the RAM. So, the Sprite (like for example 168) will only activate, when it is in the RAM, while you enter the corresponding zone. When you put the Sprite anywhere in the level and it isn't in or near your DS-Screen, it is not in the RAM and can't get loaded by the game.



The left 168-Sprite is activating the Zone-1, the right 168-Sprite is activating the Zone-0.
The yellow framed area is, what you can see on your DS-Screen, when you stand in the center of the two zones. If you stood between those zones, the yellow framed area (DS-Screen), and the 10 Tiles in every directory (Green Arrows) get loaded into the RAM. (So anything on the image)

When you enter "Zone 0" from left, it won't do anything, because the DS-Screen you see (pink framed area) + 10 tiles to right can't reach the Sprite 168, that activates something. So this one won't get loaded into the RAM. When you enter "Zone 0" from right, it will work, because the DS-Screen you see (yellow framed area) + 10 tiles to right can reach the 168 Sprite, that activates that stuff. It gets loaded into the RAM.

When you enter "Zone 1" from left, it will work, because the DS-Screen you see (yellow framed area) + 10 tiles (green arrows) to left reach the Sprite 168, so it gets loaded into the RAM and get loaded by NSMB, when you enter "Zone 1". If you enter it from right, it won't activate anything, again, because the DS-Screen you can see (blue framed area) + 10 tiles to left can't reach the corresponding 168-Sprite.

I don't know, if RAM is the right word for all that. I am not sure, if this is even true. But I AM sure, that this has some true things in it. Probably someone can explain it better or is able to find a better theory.