ImagesCollide problem

I'm using images collide but it sparks off at random points!

Its a 640x480 graphics size and this is the code to create a new enemy:

a% = Rand(1,120)
	
	If a = 5
		o.obsticle = New obsticle
		o\hnd = LoadImage("germanp.png")
		MaskImage o\hnd, 255, 0, 255
		o\kill = 1
		o\x = Rand(95,495)
		o\y = -25
	End If

Later on i use this very short thing to see if the images are overlapping:

For o.obsticle = Each obsticle
		
		If ImagesCollide(o\hnd, o\x, o\y, 0, player, px, py, 0)
			Print "SO"	
		End If
	Next

Both of these are contained in separate functions. The player co-ords involve the x changing as the player moves but the y staying the same and the enemy has the x staying the same and the y changing. The problem is, sometimes without even having the enemy on the screen it says the images have collided and sometimes it only happens when i get about 100 pixels away. But it always goes off withut the two images colliding. Its been a while since ive used Graphics mode but i don't think i have done anything wrong.

RectsOverlap and ImagesOOverlap also dont work.

Thanks for any help

Matt