The flush is weak or sluggish.

Here's what happened: when you press on the push button actuator, it pushes down on the cartridge, which starts the flush. However, in your tank, the cartridge is too far down, so the actuator doesn't reach it.

1. Loosen the setscrew (this may be located on the side or on the top of the actuator).
2. Turn the actuator counter-clockwise approximately half a rotation. The actuator should rise.
3. Replace the tank lid and flush the toilet.
4. Does it flush properly?