Where to start
If you know which one is happening, go straight to it. If you do not, the three below cover most of what actually goes wrong, in roughly the order it goes wrong.
The rule came undone and you do not know when. Most free blockers put no lock on their own settings, so removing a site is the same two clicks as adding one. Nothing was hacked. The feature was used. This is by far the most common one and it is worth reading first even if you think yours is something else.
The whole thing is gone. Chrome's extensions page belongs to the browser, not to anything installed in it, so no extension can stop itself being removed from there. Every lock on every settings screen is decorative if that page is one click away. The fix for this one is not software at all, and it takes about five minutes.
It works when you are home. This is the quiet one, and it is the one people misdiagnose for longest. A rule that can be switched off does not have to stay off for long — ten minutes on a Wednesday, back on before you are through the door. You never see it in the off state, so the tool looks like it is working and you end up doubting your own eyes rather than the software.
What these pages will not do
None of this is about catching anyone. There is no page here about reading their messages, seeing their search history, or watching them without their knowing — not because it is technically hard, but because a rule that depends on secrecy stops working the day it is discovered, and it always gets discovered.
The approach in every one of these is the opposite. Your child knows the rule exists. They can see what is on the list. What they cannot do is change it. That is a boundary you can argue about at dinner, which is a much better position than a secret you have to maintain.
The thing most of these have in common
Read enough of these and a pattern shows up: the failure almost never happens while you are standing there.
It happens at half three on a Tuesday. It happens on the afternoon you are on a train. The mechanism differs — a deleted extension, a second Chrome profile, a setting toggled and toggled back — but the timing is remarkably consistent, because that is the whole point. Nobody dismantles a rule in front of the person who set it.
Which is why "how do I make this stronger" is usually the wrong question, and "how do I fix this without being in the room" is the right one. Parentech is free on one computer and does the whole job there: the blocklist, the PIN, the class mode, all of it. What the family plan adds is reach — the second computer, a different set of rules on each, and your phone, so the fix does not have to wait until you are home.
If there is one laptop in your house and you are usually next to it, use the free version. It is not a trial and it does not expire.