There are always two snowballs. Chances are that dung beetles got to it before you did (you may have walked passed it close enough to trigger dung beetles to spawn without actually seeing the snowball). They probably destroyed it but from my past experience I can pretty much confirm that you can get snowballs to respawn if you enter/exit buildings or just save/quit. Of course this will only happen if you are absolutely sure that the other snowball has been destroyed because if it's sitting behind a tree in the corner of your town a 3rd won't respawn (remember that the snowballs can be pushed off cliffs, into rivers and into buildings by the beetles.)
Best time to make a snow person is before 5pm. You don't have to deal with the dung beetle moving the snowball that you just made. Leave it near the river or a PWP after 5pm and it could go missing.
I use the camera to check for snowballs. Just like when your playing hide n seek.
Sometimes that happens to me, as well All you really have to do is look for it, and it should be there Might be behind a tree, a shrub, a building, etc. Gotta look for it