Claiming A Step Child Without filing a Joint Return

My wife has not lived with with me for several years.  Last year I filed as Head of Household and I claimed my stepdaughter, who still lives here, as a dependent.  I received a letter from the IRS stating that I may not be entitled to some of the EITC I claimed last year.  I plan to file in the same manner this year, but I want to make sure that I am accurate.   Could I be missing something?