Thanks guys. Your suggestion worked a treat KD but I've not run into another issue.
Here's the 'code' to check if the enemy sprite has passed a certain point. It works fine until the player kills all of a certain type. For example in the picture you can see the yellow alien sprite. If the player kills all them, the game then ignores this event and the aliens go off the screen!
I've set it as an OR block so the game checks if alien1 OR alien2 OR alien3 goes passed a point but it's behaving as though it's set as an AND. Have I set this right? Can you spot any problems in the way I've used the OR block?
Thanks again.
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EDIT * - Here's the CAP file if it's any help - spaceinvaders.capx