The parts are made of plastic often show burrs and a good contact between cooler and CPU cannot always be guaranteed. For this you have to know that there are two thermal interfaces. One is the junction between the die of your CPU and heat spreader, the other is the thermal resistance between surface of the cooler and the ambient air. Let's have a look at the heat spreader that touches the die of your processor. You have to make sure that this junction has a good thermal conductivity.
Any gaps impair the cooling effect, no matter how good the cooler is designed.
The major role is played by the spring bracket that you attach to the socket in order to press the heat sink onto the die. Sometimes, micro-gaps can't be avoided. Perfect smoothness can never be achieved because any material has certain ruggedness on the surface. Furthermore, the pressure of the bracket is unequally distributed. You have to apply cooling paste to seal off any micro gaps.