It went flat
Updated
The mirror failure, less dangerous and more common as a complaint.
Gas leaves
A carbonated drink that has lost its gas tastes flat, sweeter and duller, because carbonation contributes perceived acidity and sharpness as well as texture.
Customers describe it as the product tasting wrong rather than as flat, which makes it harder to diagnose from a complaint.
Where it goes
Through the closure. The commonest route: a liner not suited to the closure or the product, or torque outside tolerance.
Through the container. PET is permeable to carbon dioxide, and a thin-walled PET bottle loses noticeable gas over months. This is why carbonated PET products have a shorter stated life than the same drink in glass.
At filling, if the product was filled warm or turbulently, so it never contained what the specification says.
Into the headspace, which is an equilibrium: a large headspace holds more gas out of solution, so an under-filled bottle tastes flatter.
Diagnosing which
Measure carbonation at filling. If it was low then, it is a process problem rather than a package one.
If it was right at filling and low later, hold retained bottles and measure over time: a steady decline points at permeation, and a sudden loss at a seal.
Check torque on the retained bottles. A closure that has loosened tells you immediately.
Fixing it
Glass or a barrier-coated container for a long shelf life.
A closure and liner specified for carbonated product, applied at a controlled torque.
Fill cold, fill gently, and fill to the target level.
And set the shelf life to what the package actually holds rather than to what the liquid would tolerate.