It take awhile for the county Assessor to catch up with the real-time values.
Also, if your home was purchased as an REO or short sale they may not allow for that in the decline of your value.
Most counties have a ssytem where you can petition for an adjustment once a year typically late in the third quarter.
Contact your local tax assesor to get the up to date procedure on how to do that. You may still have time!