The FTB Franchise Tax Board is the California Refund. They will send you a letter saying what the changed. Did you check your actual CA 540 return line 115?
I would check over your return and see if you can see anything wrong especially in the payments section. Make sure you agree with all the amounts listed. Like is the difference the same or close to the refund you got last year? Maybe you put down that you applied last year's refund to this year? Or you put down you made a payment with an extension or paid an estimated payment that you didn't make?
