I hear a lot of people saying they don't have much success with stains. There are some excellent products on the market to deal with stains, however these products are designed to deal with the chemistry of the stain.
The big problem is that the customer has usually made some attempt at removing the stain by using god knows what from under the kitchen sink.
Because of this that stain has now changed out of all recognition chemically to what it was originally. Take a tea, coffee or red wine stain which if left alone would be acidic. Sodium met such as shift or spray n go, other acidic products are ideal for removing these type of stains. But if the custy has used something like vanish or 1001 that stain is no longer acidic but highly alkaline. You then spray your acidic stain remover on to it an it is immediately neutralised by the alkalinity.
When I did stain removal for the insurance companies I tested every stain with PH paper to determine what I was dealing with. Customers lie because I would say 90% of the ones who told me they hadn't touched it obviuosly had.
If a alkaline cleaner has been used then it is a good idea to flush the stain thoroughly with either plain water or fabric rinse. This will then give you a fighting chance of removing the stain. Trying to remove the stain whilst it is full of detergent is a lot harder.