Who do I contact to find why my return was not accurate? After taxes files and paid, I received a partial refund from the state (CA), with no explanation.

You need to wait for the letter from Calif FTB saying what they changed on your return.  

Turbo Tax is a self prepared return.  It has no way of knowing or checking that you entered something wrong or in the wrong place or entered something twice  Or left something out.  It doesn't check against any outside sources.  The IRS has copies of all the W2s and 1099s your employer and banks/brokers, etc. sent in to match against.  Turbo Tax only checks the calculations of what you entered.   It is your responsibility to check it over before you file.  They only Guarantee that the calculations are right based on the entries you made.

https://ttlc.intuit.com/community/charges-and-fees/help/what-is-the-turbotax-100-accurate-calculation-guarantee/00/26355

@malebron the accuracy guarantee only covers penalties and interest if because of a Turbotax computaional error you owe more than calculated and are charged interest and/or penalties. it does not cover any additional income taxes owed because they would have been owed anyway. Nor does it cover any amount by which a refund is reduced.