I think I see what's happening. It does light up but within seconds one of my other devices also sees the message and Android stops blinking (though the notification remains).
I have the paid version of lightflow so I don't know about the free version. Remember the screen has to be off for the led to blink for notifications as well. Before I installed lightflow it did take like 5 seconds for the first blink and it was slow but now it's immediate and I set it to a fast blink rate.