Hi!
Well, it is very hard to tell via the front-end, but there are number of possible reasons:
– If you have any exclusions set, under the Advanced Options -> Exclude/include results panel, check those, sometimes there is a category, tag or date range excluded
– If there is any filter in use under the Frontend Search Settings panel. Even if the filters are not visible, they are still in effect if created! Make sure there are no filters enabled of any kind.
– In case of using the index table engine, maybe the items are not indexed? That usually happens when the products are imported via a 3rd party tool, which does not trigger the post save hook, thus the plugins does not know a new item was added, and the indexing does not trigger.
In this case, it is recommended to enable the index cron job, and re-create the index as well, so that the missing items are added.