Poo stains! oh no!

Using pocket OSFA nappies for the first time - followed instructions which say cold rinse then dry store before using half strength eco store powder. The instructions (from cherubtree) say emphatically No soaking, no napisan, no nothing.
Thing is, after 10 minutes of rinsing there were still poo stains in the nappy lining and on the insert! I thought that as I'd followed instructions these would come out in the wash but nope!
I use a flushable liner doubled over in the nappy, but my son's poo is still very liquidy so although the liner captures the 'hard bits' the poo liquid goes through.
What a mess! please tell me what I'm doing wrong! surely this isn't to be expected? it looks so horrible and after a couple of uses like this my nappies will be revolting!
thanks

thank you1!