Many of us have been having issues getting the card soldiers to show up. For me, at least, I get them when I tend to my goats and geese, and also the brown mushrooms and sometimes the white flowered bush. You can try to change the area that you're tending and see if this helps you out, hopefully you'll start seeing some soon.
We have noticed that when we NEED them, they don't show up, and when we DON'T, they do.....It is frustrating to say the least!