My AGI and income is under $40,320 and I am filing single head of house hold with 1 dependent. Why do I not qualify for the full EIC amount instead of the $15 that they are giving me?

The Earned Income Credit STARTS at $40,300-ish.  You don't immediately get the full amount at that income.
<a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf#page=53">https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf#page=53</a>

This link provides additional qualifying information on the EIC:

https://ttlc.intuit.com/replies/3299933