Bullet doesn't move with pixel precision. You can't just compare x and y of both sprites. Use distance() expression to check if two sprites are close enough, or Line Of Sight behavior with a very small radius, or collision with small collision polygons.
Note, that if Bullet speed is fast and/or fps is low, then your sprite may travel big distances between each tick and it may actually miss/skip the point where it overlaps another sprite.
That's why I recommend not using Bullet at all. Try MoveTo addon, it's much better suited for this task: