Is the object part of a group or combined? I used to have a problem where the inserted loop would appear at some location that was offset. In that case deleting history and freezing transforms would correct the bug (this btw is another gift of the QT api).

The next thing to try is save your scene, close and reopen maya and see if it works properly. I have had to start doing this recently as maya will stop refreshing my screen, or redrawing the channel box or outliner window.

You can also get a bug where you insert a loop abd forget to reset the tool settings and then undo the last insert and on the subsequent attempt to insert a loop it just drops a vert but does not draw the loop. For this you have to completely drop the tool, then reselect the object, reopen the tool and it will then insert a loop (providing you did not create a floating vert).