Very hard to say anything about this without some more information. Any error messages you're getting like timeouts?

It could also be that the combination of the 2 just don't match any records... Then try to search for a record number that is returned from the content only search to make sure that in that case you do get a result.

simple search query will return the right result as expected however it does not return anything from the combined search query and timeout isnt the issue here as it returned almost immediately.

Hi Hien Huynh 
I agree with Sander's suggestion here and the need to supply further information.
What version of CM are you using?  Was this working previously and has been impacted by an upgrade ?
If you go into the query itself what is the string query being performed,

I just tried out against CM 9.4, and the string search submitted was in the forms below, which all returned the record as expected 
idol:FFFFFFFFFFF and uri:165940                 - using IDOL query and Uri/Unique identifier
idol:FFFFFFFFFFF and number:BA....            - using IDOL query with record number
content:FFFFFFFFFFFF  and number BA.....  -  using content query with record number

Thanks Martin for your reply,

queries above were exactly what users and I used to conduct the record search. we intentionally searched the actual content or unique word or phrase on electronic documents within the container. it worked previously. Several users confirmed that. your question has lightened up something that a patching or upgrade may have caused the issue. however, what i found last few days was the advance search queries have responded very intermittent in returning the different results, such as a timeout, proper results, no record found. 

as for the incident, I may need to execute the compact process on the indexing database however i am open for any other options.

very much grateful for your and Sander's response.