The problem seems to be somewhat that the objects can't accelerate downwards, because if you switch the locations, only the red one moves.

By the way, there's no shortage of gravity simulation scenarios on greenfoot, which was something I realized and influenced my decision not to post mine.

Figured it out! It's quite obvious really, when you think of why only negative accelerations have any impact... until the acceleration is larger than a whole number. Your x and y location variables are integers, so when you add 0.05 to integer 5, you still have 5, because when you get 5.05, it's truncated back to an integer, so it just takes 5 off of 5.05. When you subtract 0.05, you get 4.95, which when truncated yields 4. The simple solution to this is to make your x and y location variables doubles, and whenever you do setlocation, do it like so: setLocation((int)x, (int)y);

When "stick" is enabled objects appear to become one object, but really they act separately, however because of this if you have three objects stuck together, there will be a wired glitch where momentum is gained even though it should be conservered.

@Danpost Could you help me figure out why my scenario isn't working please? I can see errors in the browser console, but I'm not sure how to fix it.

For not working, it may just be that you have an Object class. Try renaming it. Not sure why you used the try-catch structure.