Might be better if posted to the Zapper Support Forum as there may be more followers.
One of the issues with many zappers is the lack of proper information and instruction.
The type of zapper that you have is a single frequency zapper and using for 30 minutes may be a bad choice. It is recommended that when using single frequency zappers, the zapper should be used 3 times minimum in succession. That is 7 to 10 minutes on ( 7 minutes is a minimum ), 15 to 30 minutes off, then repeat at least 2 more times.
7-20-7-20-7 is considered to be the minimum standard. There are reasons for this.