Trying to catch queens is all ways a difficult task, here is one of the techniques that we use.

its best to keep the queen in the lower brood chamber of the hive, we do this using a queen excluder, shown below

The queen should be marked, but a handy way of trapping the queen is with an old queen cage made into a circle, with a lid on top, all the other bees leave and the queen stays inside. shown below

this gives you a queen if you can find her.