Without more it sounds like they denied one or more qualifying persons. A qualifying person for HOH status is usually your child dependent who lives in your home more than half the year. See table 4 in publication 501 for other types of qualifying person.
For example, if you claim a child as a dependent because you are divorced or separated from the other parent and it is "your year" to claim the child as a dependent, they are still not a qualifying person for HOH unless they live in your home more that half the year, even though you have the correct paperwork to claim them as a dependent for the child tax credit.
You will have to wait for a letter, or try calling, so see exactly what they did. You could also try looking at your tax return transcript.
https://www.irs.gov/individuals/get-transcript
