Question regarding entering conditional waypoints

Apologies if this is a question easily answered by reading the manual, but on my flight today I experienced an issue with the captain's CDU. I was attempting to practice some of the procedures from tutorial 2 on a route between KLAX and KLAS. I was attempting to enter conditional waypoints, but the captain's CDU was overwriting character inputs with subsequent key presses. For example, I was trying to enter a waypoint 10 miles prior to the LAS vor, but as I typed, only one letter was retained at a time. When I pressed L, it would show in the scratch pad, but as soon as I typed A, it would overwrite and delete L. If I used the FO's CDU, it worked normally. Anyone else experience this?