Here are a few things you can try if your lightbox is not displaying as expected.
Verify JS code
Assign a condition
Make sure you have assigned a condition to the lightbox. Your lightbox will not be shown if it doesn't have a condition that indicates when it should display. You can add a condition from within the Web Forms interface by clicking Add next to the lightbox.
Turn lightbox on
Make sure your lightbox is turned on. There is an on/off toggle button under the Eligible column. Be sure it is set to On.
Make sure your lightbox is published. If there isn't a green checkmark under the Live column, it means the lightbox isn't published. A lightbox must be published in order for it to display. You can publish all lightboxes that have a condition set by using the Publish button that is located in the upper right hand corner of the Web Forms Administrative interface.
An individual lightbox can be published from within the lightbox editor by clicking the Save & Publish button.
Google Tag Manager
If you're using Google Tag Manager, be sure to set your lightboxes to fire on both the HTTP and HTTPS versions of the page.
Test in incognito window
Try testing your lightbox in a Google Chrome Incognito Window. This is a helpful testing option when you are using session based rules or master rules with your lightbox.
Watch for conflicting conditional rules
Make sure your lightbox doesn't have conflicting conditional rules. Within Web Forms, it is possible to create lightbox conditions that conflict. For example, a lightbox with a conditional rule that states that it should not display unless the current date is greater than January 1, 2019 and another rule that says it should not display unless the current date is less than January 1, 2019 will never be displayed because obviously no date will ever match the conflicting criteria.