Not supposed to not find stuff. If a parcel is overlooked, they're supposed to find it later.
A common misconception:
DMG p125
'Sometimes it’s a good idea to include treasures with no associated encounter, such as a hidden cache of gold or stashed item that the characters can find with careful searching after they’ve overcome a few encounters.'
Similarly, if you fireball the 300gp painting that was part of one parcel, or let the minor quest-toting NPC that was going to give you a parcel of money die...
Or just receive and/or drink a lot of potions.
Further, still DMG p125:
'At the start of an adventure, look at the adventure in chunks of eight to ten encounters. (Include major quest rewards as if they were encounters, and if the party completes five minor quests, include those five rewards as a single encounter as well.) For each of those chunks, look at the treasure parcels on the following pages. Find the level of the characters as they work through those encounters, and note the parcels of treasure you will give out over the course of the encounters.'
It is often entirely possible to level while skipping an encounter. If that encounter happened to be the one with the dragon, and you negotiated with it, rather than killing it and taking its hoard, that can easily be a parcel lost. After all, the adventure was designed and that parcel placed there.
Or, to use an example from a game I ran, my party traded the dragon an item of their own in exchange for something from its hoard they wanted, then continued their quest. Should they get reimbursed for the item they traded away because they lost its value in play? I'd hope not...
There are no rules on those pages for replacing parcels that the party misses. Unless you're seeing one I've missed.