You could open a notepad and insert the ducky. If you see only the latest row written down then it is a delay issue.
Debug it with delay (increase it each time)
You could use a key stroke emulator, all of them will act the same, the only problem is different syntax for each one.
I would recommand https://autohotkey.com/ they are open source and free, but again, have their own syntax.
So I just finished watching "15 Second Password Hack, Mr. Robot Style - Hak5 2101" and I have a suggestion.
First of all I couldn't find any git with the initial code (on the ducky) so I have to post it here.
What I found after little research is that the command "powershell -windowstyle hidden {}" will hide the cmd.
Credit goes to stej from stackoverflow : http://stackoverflow.com/a/1802836/1997873.
If you want a littile description:
You run an empry script ==> {}
You run it with hidden winsows style ==> -windowstyle hidden
So powershell hide the main window of the process which is our cmd.
*** Tested on win10 64bit.