The fragrance disappeared
Updated
The complaint that arrives three months after dispatch, on a product that smelled right when it was made.
Strong in the drum, gone in the bottle
A cleaner that smelled strongly at filling smells of little when the customer opens it, or smells different.
The instinct is to raise the dose, which produces a product that separates and attacks the bottle without lasting any longer.
Where it went
Chemical degradation. Fragrance components are organic molecules, and a strongly alkaline or strongly acidic product attacks them. This is the big one.
Oxidation, particularly with any oxidizer in the formula, and particularly for citrus notes.
Permeation into the bottle wall. Fragrance migrates into polyethylene over months, and it is genuinely leaving the product.
Headspace loss, from a container opened repeatedly or not well sealed.
Added too hot, during manufacture, which loses the top notes to the room.
The high-pH case
Worth its own paragraph because it is the one that surprises people.
A degreaser at pH 12 is a hostile environment for most fragrance chemistry. Many fragrance oils are simply not stable there, and no dose solves it.
The answer is a fragrance formulated for high pH, which fragrance houses supply and which costs more, rather than more of one that is not.
Ask the supplier for pH stability data before choosing, not after the first returns.
Fixing it
Match the fragrance to the formula's pH, from the supplier's data.
Add at the right point and not hot.
Consider the bottle: a fluorinated or barrier container holds fragrance where plain polyethylene does not.
And test by smell at intervals on retained samples, so you know the shelf life of the scent rather than assuming it matches the shelf life of the cleaner.