Guests may start a Conversation with a Platinum or Diamond Club member for free as long as they match exactly what that Platinum member is seeking, AND if the member has not exceeded their daily quota of free Conversations that Guests can start with them.
In short, on a given day, if two other Guests start a Conversation with a Platinum member (who is looking for you) before you, then you will need to wait until the following day to try again.
If you wish to start Conversations without having to worry about such conditions then please upgrade to at least Premium.