You should buy a grout removal tool for a few $ and remove as much of the bad grout (with pin holes) as you can. Then regroup following product directions. They sell premixed wall grout.
If you don't remove any of the existing grout, you are applying a patch of grout over bad grout. It won't last as long.
Search through the posts. use keywords such as Grout and Shower.
© 2017 Renovate Your World LLC