The picker will not highlight anything
The page is probably rendering inside an iframe, or a full-screen overlay is intercepting pointer events. Use the frame selector at the top of the picker to switch into the iframe, or dismiss the overlay first — the cookie banner is the usual culprit.
Columns come back empty
Content is being rendered after extraction started. Increase the pre-extract wait in Settings, or switch the column from text to the attribute that holds the real value — lazy images often keep the true source in data-src while src holds a placeholder.
Duplicate rows
Usually a virtualised list recycling DOM nodes, or a paginator that returns page one again at the end. Dedupe on a stable column — the item URL is almost always the right key — and check the paginator's last page manually.
The run slows to a crawl
Either the site is rate limiting you, or the table has grown past what the preview can render comfortably. Drop the speed profile to Gentle and check the failed-pages tab for 429 responses. If the table is the problem, close the preview; extraction continues without it.
It worked yesterday and not today
The site changed its markup. Open the recipe, click the column that broke, and re-point it at the new element. If you have a scheduled run against the same site, update it too — the two are separate copies by design, so a local fix does not silently change what runs at 3 a.m.