When an answer finds nothing
"I know it is in my memory and it cannot find it" has several very different causes, and they are not fixed the same way. Here is the order to check them in, most frequent first.
The vault is mute
Its attention slider is at 0, so it is not queried. This is by far the most frequent cause since solo mode exists: picking a vault in the far view sets every other one to 0, and it stays that way until you bring "All" back up.
The content is intact and the vault kept recording the whole time. Raising its attention is enough.
The chat's scope does not include it
The chat only reads the vaults in its scope: the pill above your question says "All" or "N vaults". A vault missing from that list does not answer, even with its attention at 100. Check first that your vaults are enabled there.
Images follow the same rule: if your pictures live in a vault outside the scope, no thumbnail appears, and a line under the answer says how many vaults hold some. Set the scope back to "All", or add the vault.
A vault can also be in the scope without being mounted: an app's sandbox opens with the app, and a vault can still be loading right after startup. The answer then says so under the bubble, "vault not mounted"; open the app or wait a few seconds.
The vault is protected and you did not name it
A vault marked MAXIMUM is removed from questions that name no vault. The answer then comes back shorter without saying why, which reads exactly like an absence of memory. Name the vault in your question so it answers.
The file never entered
The document exists on your disk but nothing ingested it. Open "Watched folders" and check that its folder is there, and that it reads as watched rather than "Path not found".
If the folder is correct, the file may have been skipped at ingestion, for its type or its size.
Memory has no engine yet
On a fresh install the model that computes vectors is not downloaded yet. Meaning-based search then fails immediately and the answer is produced without memory. It is temporary, but it does not announce itself in the chat.
What the score does not tell you
There is no reliable "I do not know" signal in similarity scores. A search that misses and a search that hits come back with neighbouring scores. Always judge the recalled text, never the number beside it.