>.< My friend just told me about those "unrespawning" monsters.
Mushroom Cannons are taking forever to respawn as well as other hard to farm monsters. >.< Usually takes 3 minutes or more.
I find that sometimes when they're taking WAY too long to spawn (more than five minutes) it's time to head down to where the Gnoman's Land badge is, and kill the two (or one) Mushy's there.
When I head back to the "triangle" spot, the respawn times seem much faster (even if I'm only imagining it. XD).
:3 Hope it helps you out? If not, you can always try to see if killing the two (or one) mushy's to the left of the triangle area (near an entry point to the ranch) helps 'reset' the spawn times.
I have no problem with Mushroom Cannons back then because I was a little patient on farming them. Thank God I was the first to get the Mushroom Cannon Terminator badge last year during the Xmas Event. whee
I was in a worry about the Sand Castle Golems though. They took 2-4 mins of respawning. >.<