There are two common reasons for this -
The syntax used in the search may not be correct. A rogue “ or an ‘OR’ in lower case etc. may well result in no results, or unexpected results. Please check your search term carefully for anything which may be causing this
A filter used may not be applicable for a search as some search filters will clearly only give results in some areas. For example, doing a search and filtering for “Country of Funder - UK”, and then trying to find a US-based funder will not produce any results. (This issue is only usually encountered in the subscriber Dimensions versions).
Clearly, in some cases even a correctly entered keyword will not retrieve any results in a search if it is not present in the database.