I have a set of dulcimer strings by Martin & Co (the famous guitar people). Their gauges match yours as given on Jan 16th, so that's good.

I watched your video a few times. You are going pretty fast, and I could be wrong, but I think your problem is that when you play the scale on the A (middle) string, you are using the 6th fret rather than the 6 1/2.   The A scale has G# in it, so you have to use the 6 1/2 to get it.