Take a look at the “Server load limit” setting in Settings -> Link Checker -> Advanced. If it’s lower than the “current load” figure, try increasing it to something slightly higher than the current load.
It was already set as null (disabled) but I tried a big value anyway. Then Forced recheck, General tab still shows No URLs… Expanded Debug Info to see:
Resynch. flag 1
Synch. records 956
Unparsed items 956
Link records 0 (0)
<snip>
...... Skipping "page" resyncyh since all post types were already synched.
... Notifying module "dummy"
Updating server load limit settings...
Optimizing the database...
Completing installation...
Configuration saved.
Installation/update completed at 2014-03-25 12:25:25 with 26 queries executed.
MySql shows all blc_synch
records as synched: 0, last_sync: 0000-00-00 00:00:00
That debug info looks normal on its own, but it’s unusual for “link records” to stay at zero for so long. Aside from server load issues, I’m not sure what else could cause that.
The next version will include a logging feature. Once it’s out, try enabling logging and using the “recheck” option again. Then send me the log file (you can find my email address here).