Where the SYS Form Answers Refresh switch is set to Monitor Changes, this may be resolved by setting the switch setting to Full Refresh and then back to Monitor Changes.
Please see this article for guidance on updating Report switches.
Depending on the expected run time it may be necessary to do this over a weekend as it will result in an increase in the populate job run time but should result in the forms being available.
It's also possible the form is in the list but with a different name, as the list in the report contains the template_name rather than the template_version_title, meaning the original name will always be returned, even when a form has been renamed.
Running the query below, replacing <template description> with the name of the form type you're looking for.
select TEMPLATE_USER_CODE, TEMPLATE_NAME, TEMPLATE_VERSION_DESCRIPTION from DM_TEMPLATE_QUESTION_VERSIONS where TEMPLATE_VERSION_DESCRIPTION = '<template description>'