When something is not doing what you expect.
A few things look like faults and are not. They are here first because they account for most surprises.
There is a crop rectangle on the page. Overlays and crop rectangles use the same drag, so while a rectangle exists the overlays are left alone. Press Esc to clear it and they respond again. More on overlays.
Keys like D, S, T and the arrows need the editing window focused, and they deliberately behave as ordinary keys while you are typing in a text field. Click the timeline or the preview once and try again. More on the shortcut keys.
Absorbing cannot shrink the next item below a workable width on screen, and when a drag would do that it cascades instead. Because the limit is a width, zooming the timeline in lets you drag to shorter times, and Set Duration is not bound by it at all. More on resize behaviour.
The project is not marked as one. Turn on Stop-motion project under Render options. The commands stay visible in ordinary projects so the feature can be found before it is switched on. More on stop-motion.
Frame length is counted in frames, so what it comes to in seconds depends on the project's frame rate. Changing the frame rate afterwards changes every duration set that way. More on frame length.
The original has moved, been renamed, or is on a volume that is not mounted. Right-click the item and choose Replace Asset to point it at the file again; the command appears only on items with this problem.
ClapperBoard says so rather than discarding it quietly. Usual causes are a file that is not a supported type, a PDF that cannot be rendered, or a file that is unreadable where it sits — for a file still downloading from iCloud Drive, wait for it to finish and drop it again.
Supported types are still images, PDFs and audio. Video is not imported; export frames from it first. More on importing.
The project was compacted. Compacting reclaims the disk space held by orphaned files and clears undo as part of the bargain, which the confirmation states before it happens. The badge that offers it can appear mid-session as well as shortly after opening. More on compacting.
It needs at least two systems on a page to work out the spacing, so a page holding a single system is left alone. It also skips pages that are already cropped. More on splitting a score.
It needs text it can read. A PDF with a real text layer is ideal; a scan is read by recognition, which struggles with ornate type, heavy layout or poor contrast. Already-cropped pages are skipped here too. More on Smart Text Crop.
The project needs at least one image item and one audio item before there is anything to match. More on what syncing needs.
The match is between spoken words and printed words, so anything weakening either end weakens it. Check that the pages are readable, and that the recording is speech — a musical performance cannot be matched at all, and a score is timed by hand instead.
Low confidence on a page is not itself a failure: a page with little text has little to match, and is usually placed correctly by its neighbours anyway. Use Skip for pages that should not be retimed, such as a table of contents. More on reviewing a sync.
Syncing adds spacer items to keep the two lanes aligned, and reordering freely would invalidate their positions, so ClapperBoard asks before clearing them. Nothing is lost — running the sync again puts them back. More on applying a sync.
Without a purchase, exports carry a watermark and are capped at 720p; a larger resolution is scaled down to fit rather than refused. The watermark shows over the preview during playback too, so it is visible before a long render rather than after. More on the free tier.
The picture pass is the slow half. A coarser seek precision and a smaller resolution both help considerably, and are worth using while you are still checking a project rather than producing the final file.
A GIF cannot carry audio. Use MPEG-4 or QuickTime for anything with a soundtrack. More on formats.
That is seek precision. It sets how much identical picture is collapsed into a single long frame, which speeds up export at the cost of finer seeking. A lower number fixes it but takes longer to export.
If something here does not cover it, get in touch.