how do I call a Javascript function post-AJAX form submit?

Here's my simple problem. I can't call a javascript function that's included in a page after an AJAX post. I think I've encountered something similar in the past, but can't recall the solution. Here's my setup:

I have a form on a webpage that submits via AJAX using JQuery and JQuery-form. This is what it does:

1. User clicks checkboxes in the form to select any number of users to send message to. 2. User clicks submit, which triggers an AJAX post 3. respond_to js opens a hidden div generated from a partial which is displayed in a fancybox lightbox 4. The partial shown in the lightbox contains a text area with a character count which will not update when I type in the field!     - If I just render this partial on the page, the character count update works fine. I just can't get it to work post-AJAX.

I'm happy to post code, but I'm wondering if there's a general rule that I'm missing.

Any ideas?