The first place to check is the Scope Tab on the Group Policy Object (GPO). If you are configuring a computer side setting, make sure the GPO is linked to the Organization Unit (OU) that contains the computer. If the GPO configures a user side setting, it needs to be linked to the OU containing the correct user. Remember, GPOs cannot be linked to an OU that just contains security groups.
Also, Make sure you are linking the GPO correctly.
Navigate to the desired OU, to which you want to link a GPO.
Right click on this OU and select "Link an existing GPO" .
In the "Select GPO" dialog under Group Policy Objects, select the GPO you want to link and click OK
Next, check the security filtering. Make sure that the computers or users needing the policy are in a group that is specified here. Remember that domain users includes all users, domain computers includes all computer, and authenticated users includes both users and computer. By default, a GPO will be scoped to Authenticated Users.
Some GPOs make use of WMI filters. These filters can dynamically apply GPOs based on a host of factors. You want a GPO to apply if a device is attached, use WMI. However, that WMI filter has to evaluate to True for the object processing the GPO. This means that if you have a WMI checking a user only setting, you can’t scope your GPO only to computers. You can use the WMI validator Jump to check the status of a WMI filter.
Here also are the most common reason why this could not be working :
https://woshub.com/group-policy-not-applied-troubleshooting/#:~:text=If%20a%20specific%20policy%20parameter,in%20the%20User%20configuration%20section.
