For me it usually help to add little balloons mid circuit to see what signals are in different stages.
In your case, due to having all signals on the first part of the flipflop to a 1 it giver out a 0 to the second part.
Now the second part has only one 1 signal so it will stay on and give a 1 as output.
As soon as one of the input signals drop you will get a 1 out of the first part giving only one signal.
This kind of flip flop always gives out a signal, either 1 or 2 directions.
I think I may have some misunderstandings about jk triggers ...
So when the clock pulse falls, the jk flip-flop will flip?
But when I simulate this in the program, the result seems to be random, not necessarily toggling every time...why?