OT widget no longer working in popup

For over 1.5 years I have been using the Open table Pro plugin to display a reservation form in a popup in a client’s website. Suddenly the form will no longer display in the popup, but will still display on a page. I have rolled the popup plugin (Popup Maker) back far enough to rule out the problem being caused by a recent update. Any advice on how I can restore the functionality I had previously?