Updating synchronization signal

[MichaelR]

Hello

I am using STM32F412G-DISCO board. I am synchronizing 2 timers: TIM3 CH1 as master, and TIM4 as slave. Here is how I configured them:

Now I am trying to change TIM3 output compare value and thus the synchronization time, but it doesn't work well.

The code:  __HAL_TIM_SET_COMPARE(&htim3,TIM_CHANNEL_1,delayCount+64*5);

is anyone have any idea how can I overcome this issue?