The membrane should last for ~3 years. Check your pre and post RO TDS. If out is >2% of in, and you have the 75 gpd filmtec, then you may have a problem with the membrane. It could be simply a matter of it not being seated properly, or it could be a damaged membrane and need to be replaced. If the post RO TDS is fine, then what that means is your DI resin was adding stuff back into the water (rather than taking it out) when you were getting a post DI TDS of 25.