Cracking grout in a newly tiled floor is a real letdown after all the work you put into it. The causes could range from a poor grout mix to a lack of expansion joints, to movement of the tiled surface. Although it might ultimately be necessary to stiffen the floor, you can first try using caulk as flexible grout. Find out more in this comprehensive article.
How to DIY it: This job can be messy, so protect nearby surfaces by covering them with plastic or cardboard. Spray the springs with garage door lubricant (about $7 at home centers). Don’t use oil, grease, or other lubricants. They may be cheaper, or you may have them on hand already, but they won’t work as well and tend to pick up dust and grit—just what you don’t want on moving parts.