MongoDB connection pool is not working properly in Spring application -

we have created spring based application interacting mongodb end. using mongotemplate initiate connections.while running application , experiencing connection time outs below error,

com.mongodb.dbportpool$connectionwaittimeout: connection wait timeout after 1500 ms

we have changed below parameters, no luck.

connections-per-host connect-timeout max-wait-time 

our observation , whenever experience timeouts , nubmer of open connections remains same time before , after timeouts.

can please me pin point issue ?