Clans are paired using a matchmaking algorithm. This ensures that Clans of similar combined strength are pitted against one another.  Because so many factors come into play, Clan War matchmaking takes longer than ordinary attacks. 

If the matchmaking system has been searching for a while, it is best to leave it running in the background. You can still enjoy Clash of Clans in the meantime. Select the "Return Home" (1) icon in the bottom left corner of the matchmaking screen to return to your village without interrupting the matchmaking process.  

Note: pressing "Cancel" (2) in Clan Wars matchmaking will dismiss the search altogether.