Facebook Prophet add Regressor
Perhaps your repressor is for one time step behind. For e.g. if you are predicting tomorrow's stock price, then the repressor say market volume can be from today.
The other thing you can do is build a separate model that estimates the regressor for the next day and then use that in prophet. Be aware that your regressor is then becomes imperfect since it represents an estimate and not the true value. Depending on your use case, this may or may mot be a problem.