If machines are not auto joining the appropriate groups, you would want to first check to make sure it is hitting all the parameters of the search in question. One of the most common ones for the Service Plans is to make sure the agents are showing 'Onboarding Complete' as this is a criteria to be added to the group. If you have 'Enable Onboarding' checked at the Location level, this will ensure to get these agents setup when they are added to the Location. The 'Limit to Search' check box on the group level will limit the joins to 'ONLY' the machines that populate this search.
If you have an issue where no auto joins are working, you may have an issue where there are broken searches keeping the process from completing. You will want to check the LTAErrors log to see if any searches are failing due to incorrect field values or searches referencing Extra Data Fields that no longer exist. The path to the log file on the LT server: 'C:\Program Files (x86)\LabTech\Logs'. You will want to either fix the search so all the parameters are valid, or determine if the search is needed/valid and remove/modify if that is an option.