That's a weird thing! usually the script will input the string provided in the script. I don't know how it can skip a letter. Maybe there's some kind of process running in the background that kind of "lock" keyboard stroke. It can happen on web testing when the page loads images or some script executions occur while the page is loading. Maybe you could wait until you see the word "Done" on the browser's status bar at the bottom left of the window.
Can't really think of anything else at this point.
While Aaron's answer is IMO the way to go for all data entry scripts, if you wish to remain with keyed entry, you can slow you script down by going into the 'Run' section of the 'Engine Options' and changing the 'Delay between events' to a larger value. 100ms should probably do the trick. This is not a great long term solution as you are unnecessarily slowing down your script execution. The Keys function is also prone to failing if you have different caps lock or num lock settings selected when you record and play back,