Enemy Variants don't always work. Please help me fix it?

[Hidden Hands]

I have a strange problem here. And I'm going to use an example to explain it.

I have a wad of 32 completely custom levels. They were designed for DOOM 2, in vanilla format. However, I have imported all these levels into my own WAD which includes enemy VARIANTS. By this, I mean, I have the IMP from vanilla doom, but I also have a new IMP I created. Lets call it IMP2.

I boot up the game and here's what happens. As I am playing through a level, places where IMPS appear in the Vanilla version now sometimes have IMP2's. These versions ALWAYS appear in the same place each time even though the maps are for VANILLA doom, with NO custom monster variants. This is awesome, and saves me going through the levels in a map editor and adding them myself. It's perfect. However.

After MAP13 not a single custom IMP2 appears anywhere in the game. They just stop. Why do they do this? Why do they only appear in the earlier levels and how can I make them appear throughout the entire game?

[Void Weaver]

There is no telepaths, sir.

Do you can clarify what exactly levelpack did you imported into your own WAD?
Which way do you use to place IMP2 variants, via RandomSpawner?
Does vanilla imps continue spawns on the maps after 13th map?

[Hidden Hands]

I can't remember where I got the MAPS from, it was over a year ago, but they were Japanese I believe. The normal monsters continue in later maps. The strange thing is I never used anything specific. They just used to spawn randomly on their own? Like it KNEW my new IMP was a variant of the classic kind. I don't know who it knew.

[Void Weaver]

Sooo, you didn't places any custom imps on maps nor via map editor nor via decorate instructions neither, but the IMP2 however still appears on the maps, ja?
Looks there is a some kind of techno-voodoo magic works.

Without jokes now.
Does your alt-imps spawn on the vanilla Doom2 IWAD?
Do you can show full code of your alt-imps?