New photos are analysed in the background and are usually searchable within about a minute.
If it's been longer:
Check the upload finished. On mobile, large batches upload in the background. Keep the app open for a moment, or check your connection.
Refresh Search. Close and reopen the Search page. Results are briefly cached.
Try different words. Describe what's in the photo ("dog on the sofa") rather than a file name.
Looking for a person? Searching by person uses people grouping, which is off until you turn it on in Settings > Privacy.