The reject notice should tell you which dependent was rejected.  99% of the time that happens, it's either an ex who claimed the child or it's a child who filed their own return and claimed themselves. 

If it's first time for you filing that return, you might have a name mismatch.  IRS bumps returns against the SSA file.  Last name and SSN must match someone on that list.  Be careful of foster and step children and second marriages.  Names must be exact.  

Once you identify the dependent that caused the reject, check to be sure you didn't fat-finger a number, then check with the client.