Donation Page Showing as Unsecure for Donors

We use Network for good to make everyday campaign pages. We have received a few emails from supporter when they go on our donation page, that the URL is unsupportive and hackers could view private information, or that the connection is not secure and it asks them if they’re sure they want to proceed. For the majority however, there have been no issues.

Naturally, this has made some donors hesitant. Why is this happening, is there a way to stop it?