Inian sandstone discoloured at edges

[stevethespreader]

Hi what do you think is causing some of my Indian sandatone to discolour at the edges and are there any remedies to fix? Thanks in advance

 

[foxhole]

Standing water after rain?

 

[stevethespreader]

Hi foxhole im not so sure i gave the patio a light bleach down like i have done many times before but this time i have these stains on the outside of a few slabs they go away when the patio is wet come back when its dry thanks for replying