Thanks for the feedback !!!! Yes, what seems to be that some enemies can only be killed at certain speed is only my (for now) ineptitud at collisions in Game Maker or a bug (or both), sorry for that, I tried my best :(
I thought about making the ball deadly to the player as well, but I couldn't think of a good way of building the game around it, so I stuck to the timed life mechanic, and tried to build the game around throwing the ball in a good position for a combo, and then picking it up, encouraging leaving the ball out the most time possible (it gives you more points the more time you leave it out), so that there would be the tension of picking the ball back with few seconds on the clock.
The slow speed is my mistake, it's true that it feels like the player should go faster.
Again, thanks for your comment !