A few options. None are perfect.
1. Persist with the fish trap, can change the level and direction of the opening of the trap. I have had fish unwilling to swim into the trap when it was near the top of the water column, but willing to swim in with the trap on the bottom of the tank.
Some ways to encourage the fish to swim in the trap can include avoiding feeding the system for a few days and then feeding inside the trap to encourage fish to enter. May need to do this without the door to help fish become more comfortable entering the trap and associating the trap with food. Can also try placing a nori clip or alternative foods such as clams that a particular fish is more drawn to in order to encourage them to enter the trap.
2. Depending on the volume of the system, you can lower the level of water in the system effectively reducing the volume of the water the fish can swim in. Some limitations with this method. It isn't practical with a larger system. Also, if the fish dives into the rocks, may also not be effective.
3. I've used the linked technique to quickly catch fish out of my 330 gallon reef if a fish trap has not worked for whatever reason or for trap shy fish in time sensitive circumstances.
https://www.reef2reef.com/threads/m...h-out-of-a-large-reef-tank-with-a-net.907991/
Good luck!