So ... there are two things that could be going wrong. Either:
1) The Tab Key is not getting sent, or
2) The Javascript which effects the Ajax call is not getting triggered.
I presume that the number is getting entered into the text box, but can you tell if the cursor is definitely on this box when the tab character is sent and that the cursor moves out of this box (and onto the next control if there is one)?
If so then it is the Javascript which is not getting triggered. You could try sending a return character to the box, or in the IDE I htink there is a fireEvent command that you could use to trigger the Javascript event directly, though personally that invalidates your test to my mind.