If you recently installed a new LABS library and are now missing instruments you have previously downloaded, the most likely scenario is that you have installed your new LABS library to a different location rather than the original Spitfire Audio - LABS folder.
To resolve this:
1. Find the original Spitfire Audio - LABS folder that contains all of your LABS. You can check that you have the right folder by opening the LABS folder and checking the 'Samples' folder contained inside. If you open the 'Samples' folder, it will list all the LABS libraries you have downloaded. In the example below, my original folder is on external drive 'Spitfire 1', which contains my Spitfire Audio - LABS folder, with file path Volumes/Spitfire 1/Spitfire Audio - LABS. However, I have also installed a secondary LABS folder with file path Users/[username]/Spitfire folder containing only a single LABS.
Original Location with all LABS:
2. Use the 'Locate' function to point to the original Spitfire Audio - LABS folder. For steps on using the Locate function, please see this article.
3. Delete the LABS library you most recently downloaded to the secondary location.
4. Reset and reinstall the the LABS library you are looking to download. When installing, select the file path that matches the original Spitfire Audio - LABS folder to install to the correct location. For steps on resetting and reinstalling, please see this article.
If going by the above example, when I reinstall the the LABS library it should match the original file path in the Spitfire Audio app. Notice how the path below matches the Volumes/Spitfire 1/Spitfire Audio - LABS path of the original LABS libraries.