Wysiwyg, ckeditor disappers when click "Add another item" button I use Field collection modules to group image + textarea for multiple field group inputs.
When I click "Add another item" button, ckeditor bar disappeared from the textarea.

I think there would be some errors occured on Ajax request for an iframe tag appeared at the bottom of the script.

This thing only happened when field collection group contains both image and textarea.
Field collection with only textarea are fine.

The modules I use:
wysiwyg 7.x-2.1
ckEditor 3.6.4.7575
IMCE 7.x-1.5
File Field Sources 7.x-1.4
field collection 7.x-1.0-beta4
I had this problem as well. This was fixed in http://drupal.org/node/1155678#comment-5358290 by adding a detach function to the WYSIWYG behaviors. You can apply the patch at the above link (that's what I did) or you should be able to download the 7.x-2.x-dev version (I haven't tested that) since it's been committed already.