I had this issue also but since my unit it's out of the machine at the moment and I can see what going on when more then one shelve drops. I can up with the following: Note assuming all springs and shelve mechs look in good shape.
When you reset the shelves in a column they all need to be dropped, if you try to reset just some it will cause the latching mech not to set correctly, each shelve is dependent on the shelve above it to set correctly .
I can get multiply shelves to drop in any column if I set them wrong, also if you are not filling the column completely, keep filling it going up until all the shelves are dropped or use the free vendor switch to drop the remaining shelves and then reset the whole column.
Not saying there aren't other issues that maybe causing problems, if you shelves drop randomly this may be the fix to try.
Ray
