STM32 program to control RGBW leds and be sync

I need to sync the string of LEDs to work at the same time when a new signal comes from the Master. I have 5 strings of LEDs but when it does the blinking the 5 strings are not synchronized. It has a Master and 5 slaves. Each Slave control a led string that has a different amount of LEDs. The data sent from the slave to the string of LEDs is by PWM DMA using the NRZ protocol.