Dear Samantha:
Ah, slate!...
By saying slate you're saying very little – if anything at all. There are many different (and I do mean different) kinds of slate that span from dense and acceptable rocks, to slate that will actually fall apart under running water or turn into mud, and everything in between.
So which slate do you have?
That said, I would suggest you to clean it really well with a heavy-duty alkaline cleaner with chelates in its formula like MB-3 (so to remove any possible soap film and mineral deposits from its surface), and then, hoping that nobody applied an impregnating sealer to it, treat it with a good-quality stone color-enhancer like MB-6. That should a long way toward the solution of your problem, unless of course there are structural problems, like some bad install.
May I ask you now to please read and e-sign our Statement of Purpose at:
http://www.marblecleaning.org/purpose.htm?
Ciao and good luck,
Mauri
Article ID: 1224
Created On: Mon, Feb 18, 2008 at 1:03 PM
Last Updated On: Mon, Feb 18, 2008 at 12:00 AM
Authored by: Maurizio Bertoli [mail@mbstone.com]
Online URL: https://marblecleaning.org/knowledgebase/article.php?id=1224