Table of contents removes 'levels' when 'editing'

[MelS]

Hi
I have a table of contents, using the following styles and levels:
Style Level
Appendix 5
Heading 1 1
Heading 2 2
Heading 3 3
I have no problems updating/refreshing the table of contents. However, if I ‘Edit Field’ ‘Table of Contents’ the styles ‘APPENDIX’ and ‘Heading 3’ have lost their level (in the 'table of contents options' dialogue box) and are the removed from the TOC in the document. I have to re-enter their levels everytime i edit the table.
There are numerous users of this template and I require the table of contents to have all the above headings/levels. Can anyone advise why, when I edit the TOC, I loose Appendix and Heading 3 levels?
thanks