Enable trace level logging. You will get a bunch of message that should tell you what it is doing. Stuff like
[2019-01-10T16:39:51,066][TRACE][filewatch.tailmode.handlers.createinitial] handle_specifically opened file handle: 85, path: foo.txt
[2019-01-10T16:39:51,093][TRACE][filewatch.tailmode.handlers.createinitial] add_new_value_sincedb_collection {"position"=>0, "watched_file details"=>"<FileWatch::Watched File: @filename='foo.txt', @state='active', @recent_states='[:watched, :watched]', @bytes_read='0', @bytes_unread='954', current_size='954', last_stat_size='954', file_open?='true', @initial=true, @sincedb_key='59521 0 51714'>"}
[2019-01-10T16:39:51,130][TRACE][filewatch.tailmode.processor] Active - file grew: foo.txt: new size is 954, bytes read 0