There are a few probable causes but before you tear up the tile and start over try this quick cracked grout fix.
How to fix cracked floor tile grout.
4 spread the grout and completely fill the cracks using a grout float.
A cup or bowl should work fine.
If you have wider cracks in the grout or missing grout then it s time to consider removing the old grout and installing new grout between the tiles.
Use a bamboo skewer and drag it along the grout line.
In order to repair damaged grout that s cracking using new grout follow these steps.
You can use a simple grout removal kit or i prefer using a dremel 568 grout removal kit which attaches to my dremel tool or the rockwell sonicrafter.
While grout may crack from wear and tear an uneven or weak subfloor.
While grout is a durable material tile surfaces suffer a lot of wear and tear that can crack it resulting in a shabby look.
Is the grout between your floor tiles cracking or worse crumbling away.
Once the grout cracks for any reason it s time to repair broken grout before there is any damage to the surrounding tiles.
Mix the new grout with water in a small container.
Use a vacuum with a hose attachment to clean out any excess loosened grout.
This will loosen any cracked and damaged grout.
Repairing wide cracks in grout.
It is best to do this at an angle then smooth over the grout with a wet finger.