Ensure wl-clipboard, grim, and slurp are installed (see Installation → Build from source if you are not using packages).
Check your config under [capture].
On GNOME, portal capture may ask for permission and can be slower. If freeze does not start, check the desktop portal service.
On newer Sway/wlroots stacks, update Wayscriber before debugging deeper; older builds could bind a screencopy path that did not provide the shared-memory frames Wayscriber needs.