Help! Trigger problems!

I need help fixing a bug in one of my maps.
It's a remake of Zergling Blood, and you win by killing 2000
zerglings. Every 200 kills you get a hero.
I set it so that the player starts with 199 kills for easy
testing.
I kill the next unit, and the game gives me a Selector.
I fly the Selector(a levitating Goblin Sapper set to Peon status
so you can see how many you have on the left side of the screen)
into the region that should trigger giving me a minigunner hero,
but nothing happens.
Here's a layout of the trigger that pops up the dialogue for
giving a Machinegunner(which never appears! :():

Machinegunner Select
--Events:
------Unit - A unit enters Choose Machinegunner <gen>
--Conditions:
------(Unit-type of (Entering unit)) Equal to Selector
--Actions:
------Set selectplayer = (Owner of (Entering unit))
------If ((Number of units in (Units in Choose Machinegunner
<gen>)) Greater than or equal to 1) then do (do nothing)
else do (skip remaining actions)
------Set selectorunit=(Random unit from (Units in Choose
Machinegunner <gen> owned by Player 1 (Red)))
------Dialog - Change the title of mgdiag to Do you want a
minigunner for one selector?
------Dialog - Create a dialog button for mgdiag labelled Yes
------Set mgdiagyes = (Last created dialog button)
------Dialog - Create a dialog button for mgdiag labelled No
------Dialog - Show mgdiag for selectplayer

It never displays the Dialog, though!

try deleting this part... maybe don't solve your problem but anyway you don't need that... because if a unit enters a region A of course there will be 1 or more units in region A....

Doesn't fix it.