A deposit return machine that rejects a container is usually not broken. It is applying a set of checks that are stricter than most people expect and largely invisible from the outside.
The machine is verifying a registration, not a shape
Each deposit-bearing container is registered in a central database against its barcode. The machine reads the code and looks up whether that specific product carries a deposit and how much.
A container from outside the system has no entry, which is why imported drinks and older stock are refused even when they look identical to accepted products.
Registration also expires. A discontinued line eventually leaves the database, and containers found later in a cellar are then rejected despite having carried a deposit when purchased.
Reading the barcode is harder than it looks
The container rotates on rollers while a scanner searches for the code. A crushed or torn label breaks the pattern, and the machine has no way to guess the missing digits.
Condensation, glue residue and shrink wrap that has slipped all interfere in the same way. So does a label applied over a seam, where the code curves sharply.
This is the most common cause of a rejection that seems arbitrary, because the same bottle presented a second time with the label straightened is often accepted immediately.
Shape decides which stream it enters
Machines separate single-use containers from reusable ones, because the two are handled completely differently afterwards. Reusable bottles are sorted by their physical profile so they return to the right filler.
A profile that does not match anything in the machine's catalogue cannot be sorted, so it is returned. Unusual promotional shapes are refused for this reason more often than standard ones.
Weight and volume are checked at the same time. A container that is heavier than expected suggests liquid or foreign material inside, which the system treats as a reason to stop.
Contamination is a real constraint downstream
Single-use plastic is compacted and baled for recycling, and the value of that bale depends on how clean it is. Residual liquid and food waste degrade the whole batch.
Reusable bottles go for washing and refilling, where contamination carries a direct hygiene consequence. Anything used to store fuel, paint or chemicals must never reach that line.
The machine cannot inspect the interior, so it relies on weight, sound and the absence of a cap as proxies. Those proxies occasionally reject perfectly ordinary empties.
Why the machine stops entirely
Behind the slot sits a compactor, a sorting mechanism and bins that fill during the day. When a bin is full or a jam is detected, the machine halts rather than risk damage.
Staff clear these interruptions between other duties, which is why queues form at predictable times such as late afternoon and Saturday morning.
Returning containers outside those peaks, with labels intact and caps removed according to the posted instruction, avoids most rejections without any understanding of the machinery being necessary.