If you remove the drive from the case and turn it upside down, you should see 2 black "arms" that form a "U". These have posts that are hit by the DVD when it enters the drive. With the ends of the arms facing away from you, look at the lower right arm extension. You should see 2 square parts that may be gold in color. These are the micro-switched that trigger the load/unload process. I would verify that these are working properly. With a continuity tester, push in each one to see if it works. I suspect that one of these has failed.
Report back here with the results.
Dan