How do I make Heading 2 content be part of ToC?

I am using LibreOffice version on Ubuntu 18.04.10. When I try to create a ToC it only registers Heading 1, which is the main heading for the chapter. My subheadings are all under Heading 2 style, but they are not being picked up by the Toc.
I have checked that the main text is styled Text Body, and gone through each subheading, making it first Text Body and then putting them back to Heading 2, but the subheadings still do not appear in the ToC.
When I am inserting the ToC I notice the example of the levels are written as Level 1.1 and Level 1.2. I suspect this is not significant. The document is saved as .odt. I have a copy saved as .doc and the ToC seems to be working as normal there.
Any suggestions?

As you have two documents, you cannot infer good behaviour from one to put the blame on Writer for the second one.

There a few things to check:

  • when you Insert>TOC & Index>TOC, Index or Bibliography, field Evaluate up to level sets the limit of collection

    By default, it is set to 10, but if you changed for 1, only Heading 1 are included.

  • you may have removed Heading 2 from the TOC machinery

    Tools>Chapter Numbering, Numbering tab defines which paragraphs participate in the TOC. Click on Level 2 at left/ Paragraph style: tells which paragraph style is associated automatically to level 2. If you don’t read Heading 2, fix it.

As long as Heading 2 is associated to a TOC level, you can’t change the style Outline & Numbering properties. Thus, even if you customized the formatting, you’re quite safe here.

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Thank you,.
You were quite right, I had somehow completely lost Heading 1, 2 & 3 from the list of levels. When I restored them under Tools>Chapter Numbering, Numbering the ToC build itself with no problem.
I am most grateful.

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Hello @Tony45

I do not understand

When I am inserting the ToC I notice the example of the levels are written as Level 1.1 and Level 1.2. 

Could you edit the description of your question and provide details about this ?

When the TOC was created, in the eponymic dialogue, pane Type what was the max level of evaluation of the levels : 10 will cover the whole titles and sub-titles; 1 will only cover the first level ?

During the sale creation, was Entire document selected for Create the Index or Table of Contents ?

In the pane Entries does the settings for levels 2 and following are identical to the one for level 1 ?

In the Styles pane, is the Assignment list provide a style for every level ?

in the style list, opened by F11, do you see all heading levels listed below the main entry Heading, in the proper order ?

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Kind regards, Michel

Thank you Michel.
What I was trying to express was when I open the eponymic dialogue, pane [I love the arcane language ICT has generated] on the left there is an example of how the Contents will appear. It is in this example that Heading 1.1 and 1.2 are seen, but it turns out to be irrelevant. I had somehow switched off Headings 1, 2 and 3 from the Levels list. Thanks to another response to my query I have put that right, and all is working now.
Thank you for taking the time to respond and offer such helpful suggestions.