Sometimes the hitbox doesn't work right especially around the screen edges, maybe you could define minimum and maximum X and Y for the targets to spawn so they don't show half off-screen, you could also probably have a random chance for a target to have a sine movement up or down, that would increase the difficulty a little bit by making the target move. I don't really use scratch but I'm sure these wouldn't be too hard. Overall not too bad for a first game, I hope this helps!