Why doesn't spotting enemies always work?

Solution 1:

When the game was first released there was no limitations on spotting and you were able to spot through some non-transparent objects (trees). This caused some major outcry and was fixed in a patch by adding spotting overheating and inability to spot through the objects (which now often makes spotting through the leaves problematic even at direct sight).

Spotting, like the repair tool, uses "overheat" to prevent excessive Socialize use. The player is capable of about five Socialize button presses in rapid succession before overheating and being inoperable for five seconds. With the cooldown, the fastest continuous rate that a player can spot without overheating is once every two seconds.

[source]

Solution 2:

There is definitely distance limit. Very often I can see campers much further, then I'm allowed to spot them. This is especially annoying on big, open maps like Heavy Metal or Harvest.