What type of liners are you using ?
I have a few different ones some are stay dry, some are silk(more for skin integrity than protecting the nappy) and some are I think micro fleece maybe I'd have to check.

Are you taking any vitamins? I would use some vinegar in with the wash and see if it helps. Sun would bleach them out too ( not easy in June though!)
Nope no vitamins either, although I probably should be lol