I noticed our SHS-P718 has been doing some strange things over the past couple of weeks, including unable to open from inside and random failure to lock/unlock like it was dead batteries, even though I had just replaced them. I had a good look at it today and thought perhaps the batteries were not contacting properly and the lock was intermittently losing power.
I gave all of the battery terminals on the lock a good clean with a bit of fine sandpaper and then stretched the springs slightly with a small screwdriver. Problem solved, the door lock is working great now!