Parental controls periodically not working on my kids' phones

Hello, I was speaking with someone last week regarding this issue but I can no longer  access the conversation. My son's phone usage will stop being monitored for no reason. I see it again this week; I got my weekly report and it shows him using his phone early in the week but then it stops on the weekend when I know for certain he has been using it. Please tell me what we can do to fix this. We shouldn't have to keep restarting his phone to make sure his monitoring works. Thank you.