Reduced Test Cases

Step 1: Isolate the bug on your end

More often than not, the problem is with your code, not with FullCalendar. To determine this, remove as much of your code as possible:

For more info on this technique, read Reduced Test Cases.

Step 2: Post a public demonstration of the bug

Use a service like Codepen or JSFiddle to demonstrate the bug online. Enter your JS, CSS, HTML, and JSON and wire up all the relevant dependencies. To help you get started, here are some debugging templates:

FullCalendar v4

FullCalendar v3