We're not aware of any GPS issues in our PA28s with the latest builds. My first suggestion would be to complete a fresh reinstall of the affected product(s), ensuring that you have the latest version installed. The latest version of the Arrow III is v0.11.3.
If the issue persists, then I would recommend trying this with only the Arrow III in your Community folder. That will then tell us if there is a conflict with another mod.
If you rename your Community folder something like "Community.old" and then create a new one named "Community" and run the Arrow III installer, that will allow you to test this with an empty Community folder and then revert back to your old Community folder without having to reinstall everything. If the GPS powers on normally without anything else in your Community folder, then that points towards a conflict with another mod in your the "Community.old" folder.
If the above solutions don't work, could we kindly ask you to raise a ticket with Just Flight Support at the following link, and one of our support team will be able to assist with further troubleshooting: https://www.justflight.com/support
Mark - Just Flight