Stained inserts

Mi

I’ve been cloth diapering for about a month or two now, so I’m still fairly new to it. We use pockets and inserts. I know half of the inserts are bamboo and I’m honestly not sure what the other ones are. I have a handful of inserts that got pretty badly stained and despite laying them in the sun, will NOT come out!! I use arm and hammer with oxy clean detergent. I do a cold pre wash then a hot main/super wash. After washing they don’t stink or have any residue, in fact they smell quite nice. they’re just really badly stained. Any tips to get the stains out? Is there any possible way to prevent the stains in the first place? Winter is coming up soon and I will not have any chance to lay diapers in the sun, so I’d like to find another routine to try and prevent or remove stains without sunning.