This is a known behavior under the current version of the LegaSuite client software.  It has to do with "focus" - if your cursor is already on that field, then it has "focus" and you can click the drop-down list and make your selection.  If that field doesn't have focus already, then your first click moves the cursor to that field, and the second click displays the drop-down list.

We have been told that this will eventually be changed in a future release of the LegaSuite client software.