What is considered a substantial change in circumstances to modify custody?

First of all, when did you make the first request? Because if it was right after a divorce decree was signed by a judge or an initial parenting plan was put in place, the court probably is going to require at least a year pass before it will reconsider any of the terms of the agreement, unless there is some kind of emergency or life-threatening situation that comes up in the meantime. Change in circumstances must be something substantial, like one or the other of you is planning to or has moved out of state with the kids, gotten a new job or promotion that takes you/them away from the child(ren) for long periods of time, etc. If you have proof of a considerable change or changes, then petition the court for modification. Best of luck!