Alpha Merge Image not showing up in the 2D viewport

Hey all. I'm following one of the official designer tutorials and I'm having some inconsistent results. For example, when I use the alpha merge with a color and my greyscale arrow, only the solid colour shows up in the 2D viewport. In the tutorial, the arrow shows up. Any idea what I'm doing wrong?

Here's a screenshot to illustrate the issue. Any help is appreciated.