Excuse me for asking dumb questions, but have you fought Scar and Tank yet? That was the problem for me. You can see how to fight the two on the guide if you haven't. Assuming you haven't already unlocked the bridge yet of course. From there just choose "Find another way" when you're on the bridge to get to the cave. Explore the cave and you should find the glowshrooms. Then again, if you have done all of those things you could just redownload the game and transfer your save if it helps.