I'm not sure what you mean by you're head of household. you can not claim yourself or your spouse as a dependent.
you can't claim your parents unless they filed a joint return only to claim a refund of income tax withheld or estimated tax paid. In other words, they had no income tax liability. their filing blocks your e-filing even if you can claim them as dependents because their SSN was been used on their return.
other requirements to claim your parents as dependents
A) each must have gross income of less than $5050
gross income includes
1) all taxable income
2) gross rental income
3) for schedule C gross profit (but can't be negative)
see 1040 instructions for the complete definition of gross income - at the bottom of chart A on page 9
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
B) you must provide over 50% of support for each of them
