It does not do anything: Please double check that you set variables before using them. Crossover sets both offsprings in a hidden way, since blockly does not support more than one return value. If you're not using crossover, you need to set the offsprings yourself or use the parents directly.
It stops before finding the optimum: Please increase the maximum rounds in the central 'while' loop.
It crashes (as in stops with an error): Please download the XML file of your algorithm and send it to me: jellyfishcoding at live.de
It crashes (crashes browser completely): Please make sure you don't have any infinite loops in your code.