you should get a letter of explanation in about 2 weeks from the date the return was processed. it will have a phone number to call for more info. if you want to find out sooner, you'll have to call the IRS. best guess is that the IRS determine the dependent wasn't a qualifying individual for the HOH status.
Call 800-829-1040
1) Choose your language,
2) Then choose option 2
3) Then 1
4) Then 3
5) Then 2
6) When it asks you to enter your SSN or EIN do nothing – wait - - After it asks twice, you will get another menu
7) Then 2
😎 Then 3
It should then transfer you to an agent. Note when its phone line are busy you may be on hold for a long while or disconnected
if you want you can use this IRS website to check your filing status.
https://www.irs.gov/help/ita/what-is-my-filing-status
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.