Do you have the full rejection report?  If not, highlight the rejected file in the EFCenter, then hit F12, that opens the Efiling Reports, click Rejected returns and eitehr view or print the report.

Youre looking for this line...the dependent number will be in the square brackets [  ]

This screen shot is for a TP that had a reject for 2 of their 3 of their dependents (the first and second ones in the list of dependents) be claimed by the other parent.