You need to right click on the affected paragraph, select Bullets and Numbering...
Ensure that you are using an Outline Numbered setting and that the Continue previous list option is selected.

If this is already set up correctly then click on the Customise button and ensure that the Number format box has grey highlighted numbers - this denotes that they are using a Previous level number setting

If you can't see any grey highlighted numbers then you can recreate it as follows:
Delete all the text in the Number format box
Select Level 1 from Previous level number drop down
Type . (or whichever separator you wish)
Select Level 2 from Previous level number drop down
Type . (or whichever separator you wish)
Select 3 from the Level list
Select 1, 2, 3, ... from the Number style drop down
Type . (or whichever trailing you wish)