There are three showers in my house: two have had low pressure from their showerheads since moving here, so we only use one of them.

I recently decided to try to get to the bottom of the low pressure issue in the other two. I first took the showerheads off of all of them, and noticed that the flow from the gooseneck itself was low, ruling out the showerheads as the reason for the pressure difference. I then tried replacing the cartridges to no avail. With the cartridge out of its socket, I opened each valve (the screws for cold and hot) and quickly saw that there was ample flow out of the front of the valve assembly (see photo if my description is not doing it justice). Therefore, I'm a bit puzzled as to where the issue is.

I attached some photos of the flow and valve assembly. Please let me know if photos of other portions of the setup would help. It's worth noting that the "good" shower has an identical cartridge and valve assembly. I can provide pictures if needed. Thank you so much for the help!