Is unreliability some general thing with parental control software? Why?

Dear support forum, 

I have tried for a whlle to set up a working time limit solution on my child's computer. I've tried Norton Family, Verity Parental Control, Romaco time control and some others. None of these give a reliable performance. All seem to have similar difficulties despite working in totally different ways (Norton for instance is the only one that monitors computer use on several computers; Verity sets up a server on the child's computer that you can use for controlling time limits from another computer; Romaco is simpler and needs you to set it up on each account that you use it on). That is, some times the program starts up/works properly, and some times, at irregular intervals, it doesn't.

 

Does anyone have a clue why? It seems very strange that I'm unable to find a single program that can do the job. If this forum wasn't full of people having exactly the same trouble (although only with Norton as far as I know) I'd seriuosly suspect that I'm the  one doing something wrong...

 

My child's computer is running windows 7 btw.