spawn trigger

i really need help know i am trying to make a td and i am nearly finish,
and i am having some problems whit some triggers like wait 30 seconds doesnt work. so i dont know how to spawn those creatures then and i have tryed like 5 different triggers to spawn and non of they wont work so could someone give me a trigger that will work??

Wait actions don't work well in loops (such as for loops or pink unit groups). Also, if the trigger does anything wiht "Last created unit" or something similar then it will screw up. The best way to do it is to have the wait action at the end of the trigger followed by an action that runs that same trigger again.

there is no last created unit and the time is at the end of the trigger but i will try what u said