Thanks for the reply,
Strangely the previous issue didn't not appear again, but I got another issue.
To do an advanced search using metadata, when I enter 'Metadata type' and 'Metadata value', which doesn't match an existing metadata value the search returns some values having a different metadata value.
For example, if I have a metadata called region associated with some documents with a value 'north', all documents which have this metadata with different metadata values are returned after a search.
Thanks again!