Unexplained InnoDB timeouts

I've been seeing some very basic updates timing out lately and have not been able to determine the cause. An example: //# Query_time: 51 Lock_time: 0 Rows_sent: 0 Rows_examined: 0 UPDATE photos SET position = position + 1 WHERE (photo_album_id = 40470); The same log has no entries with a Lock_time > 0. Running show innodb status does not reveal any related locks either. This problem appears to be affecting at least 5 different tables based on my app server logs (which show a Mysql::Error: Lock wait timeout exceeded error related to each corresponding entry in the mysql-slow log). Any idea on where to go from here? I'm hitting dead-ends in all directions. Thanks. EDIT:

CREATE TABLE photos (
  id int(11) NOT NULL auto_increment,
  type varchar(255) NOT NULL,
  photo_album_id int(11) NOT NULL,
  user_id int(11) NOT NULL,
  title varchar(255) default 'Untitled',
  description text,
  credit varchar(255) default NULL,
  photo_file_name varchar(255) default NULL,
  photo_content_type varchar(255) default NULL,
  photo_file_size int(11) default NULL,
  photo_updated_at datetime default NULL,
  position int(11) default '0',
  views int(11) default '0',
  folder varchar(255) default NULL,
  published tinyint(1) default '0',
  published_at datetime default NULL,
  created_at datetime default NULL,
  updated_at datetime default NULL,
  album_published tinyint(1) default '0',
  comment_count int(11) default '0',
  audio_file_name varchar(255) default NULL,
  audio_content_type varchar(255) default NULL,
  audio_file_size int(11) default NULL,
  audio_updated_at datetime default NULL,
  cover tinyint(1) default '0',
  slug varchar(255) default NULL,
  comments_count int(11) default '0',
  delete_from_s3 tinyint(1) default '0',
  batch int(11) default NULL,
  audio varchar(255) default NULL,
  PRIMARY KEY  (id),
  KEY index_photos_on_album_published (album_published),
  KEY index_photos_on_batch (batch),
  KEY index_photos_on_comment_count (comment_count),
  KEY index_photos_on_created_at (created_at),
  KEY index_photos_on_delete_from_s3 (delete_from_s3),
  KEY index_photos_on_photo_album_id (photo_album_id),
  KEY index_photos_on_published (published),
  KEY index_photos_on_published_at (published_at),
  KEY index_photos_on_type (type),
  KEY index_photos_on_user_id (user_id)
) ENGINE=InnoDB AUTO_INCREMENT=42830 DEFAULT CHARSET=utf8