Assuming the system itself has no malfunctions, the most likely reason is signal interference. The only time this has happened to me is when I had my keys in the same pocket as my cell phone. I'm in the habit of making sure my keys are in a separate pocket then my phone. Others have reported instances of signal interference depending on location (nearby radio towers, etc.).

Maybe another reason is low battery in the key. Not sure how low the battery has to get before it triggers the warning on the dash, but perhaps its just low enough to cause occasional issues but no warning. Experiment by positioning the key closer and closer to the handle. If that still doesn't work, move to the passenger side and see if it works fine on that side. If it does, maybe the antenna in the driver's door handle has a loose connection and opening and closing the door closes the loose connection.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
Distance: 36187km (22486mi) over 24 mths (ave trip 20 min)
FE: 6.80L/100km (34.6 mpg)
Dealer repairs reqd: none to date