Computers cannot actually create random numbers, instead they perfom a psuedo-random function based on a defined seed that is so complicated the numbers appear random.
If this seed is consitent between gameplays however, the exact same sequence of random numbers will play out.
Gamemaker proivdes the function randomise() to automatically change the seed each time.
Where should randomise() be called?
randomise() is a one time function which should be called in a Create event or similar.