I have a $27+k deduction relating to the car used for my business, why does that deduction not give me a greater refund?

I have been deducting mileage expense for my sales job for several years.  i got divorced and started filing single head of house hold this year and the prior 2 years.  i noticed as i progressed through the process of completing a return that as i input the info for mortgage interest and state taxes, my in refund would increase after each entry.  prior to me filing as single head of household, when i entered the mileage expense, i got a significant bump in the refund...not any more.  is this because i'm not filing as married with dependents???