This can be due to various reasons such as time of day & the amount of players online (not currently in a match), search preferences and connectivity. You may want to try changing the search preferences. If the search preference is set at Best, it may take a bit longer to find a match.
Additionally, you may want to establish a static IP address and port forward. Here are the port numbers you will need Activision Support . Should you have any questions on how to port forward, please contact your ISP or the router manufacturer. Also note; a wired connection is preferred for online gaming in order to sustain the connection quality.
Here's another support article on how match making works you may find helpful Activision Support .