I was trying to find them underground but I Searched so long by those glitters on the map but I cant find a glitter in the wall is that a bug
Lmao I was stalking my Sister's account (Phoenix447) and saw her half-answer, and promptly decided this is my chance to one-up her, so here I go.
Yes, this is a bug. This happened to me multiple times during my Oak Challenge. Here are some solutions that worked for me:
However, you can get more shards in Fuego Ironworks and in the safari.
Fuego Ironworks - Exchange Star Pieces for 1 of each Shard with the man near the smelting furnace.
At the safari, go in. In your right there's an Ace Trainer, every day he will give you a shard.
Source
Have you frequently touched the touchscreen? That should allow for those little glittery spots indicating an item to appear on the walls or on the top map. Try moving around a bit more and maybe leave and return underground to refresh. If that doesn’t help, try restarting your game, and make sure your touch screen is working correctly.