C172 pilot is unable to command his auto pilot to descend too step down fix altitude nor to minimums during practice GPS-B approach to HWD. Reporter passes over airport at 2000 FT.

Narrative

I was in contact with NORCAL TRACON requesting a practice GPS-B approach to HWD. NORCAL cleared me for the approach and upon established; descended to 2;300 per the chart (between imply and DECOT intersections). At DECOT Intersection; I commanded the autopilot to descend to 1;500 FT. The autopilot descended to 2;000 FT and did not descend any further. I then was looking at my other instruments and then asked to contact HWD Tower. At 2.0 NM from MAPVV; I commanded the autopilot to descend to the MDA and noticed that the altitude was still 2;000 FT. I reattempted to command the autopilot to descend but it did not respond and the autopilot continued to hold 2;000 FT over the field. After approaching the MAPVV; HWD Tower issued a descend to 1;500 FT and continue southbound then to re-establish contact with NORCAL TRACON. I believe that there was too much reliance on the autopilot and not recognizing that it may not have been in the right mode (vertical speed) when issuing the descend command. Situational awareness was lost when I was trying to figure out what was wrong with the autopilot. To correct this situation; I should have disengaged the autopilot and hand flown the approach into HWD upon realizing that I was too high at the 2.0 NM to MAPVV.