Low pressure in shower only - tub fine

Hello masters of flow...

So I changed the valve stems in my tub a couple of months ago (2 temp + 1 diverter, all rotary), and ever since then, the water pressure in my shower only has gone way down. Tub is fine. Nice heavy flow just like before. Diverter works, but shower head pressure is way lower than before. Any idea what this could be?

Context: I bought a 3-stem kit from HD, which included the stem socket... I have no idea what brand the original stems are. Also, I have pretty hard water, so I'm wondering if I just loosened a blomp of mineral deposit when I pulled/replaced the old diverter stem. I also did *not* replace the seats for the stems. Could that be it? Could it possibly be that I'm a total newb-tard and had no business even trying?

Appreciate any help. I also admit I didn't search first. I'll do that now & if I find the answer, I'll take this down.

TIA y'all!