It's not really that common. We as a society have a tendency to focus on the negative, to focus on the individuals who are not paying. For every one dad, or even mother, who is not paying child support, there are hundreds of fathers and mothers who are indeed paying child support.
How common is it? It happens, but the overriding factor is most parents who are obligated to paying child support do indeed pay. If they indeed fall upon some kind of financial difficulties, they openly make up those deficits.