UART handler in Server light side isn't getting called

Hello Team,

I am working on nRF52833 DK for a custom work.

We have 2 nrf 52833 boards( call DK1 and DK2)

DK1 runs a Light Switch Client

DK2 runs a Light Switch Server

Our custom setup includes another development board say DK3 (based on Rockchip) which is connected to DK2 through UART module.

Now the requirement is like. some bytes of data (ex: 6 bytes) will be sent from nrfConnect to DK1

DK1 receives it and sends it to DK2 through BLE Mesh

Once after DK2 receives it, it will send the received data to DK3 through UART interafce

Till the above part its working fine.

Now as per our requirement, some bytes(say 16 bytes) will be sent back from DK3 to DK2 through UART.

What we expect in DK2 is to receive the data through UART IRQ handle function. "uart_event_handle"

but that wasn't happening. 

The UART handle has been registered using LOWEST priority thread level.

Below is the snippet.

    APP_UART_FIFO_INIT(&comm_params,
                         UART_RX_BUF_SIZE,
                         UART_TX_BUF_SIZE,
                         uart_event_handle, /*uart_error_handle,*/
                         APP_IRQ_PRIORITY_LOWEST,
                         err_code);

And below is my interrupt handler code snippet

void uart_event_handle(app_uart_evt_t * p_event)
{
    static uint8_t data_array[BLE_NUS_MAX_DATA_LEN];
    static uint8_t index = 0;

    uint8_t rec_byte = 0;
    uint32_t       err_code;
    __LOG(LOG_SRC_APP, LOG_LEVEL_INFO, "%s : UART EVENT HANDLE \r\n", __func__);
    switch (p_event->evt_type)
    {
        case APP_UART_DATA_READY:
         UNUSED_VARIABLE(app_uart_get(&rec_byte));
        

This interrupt handler isn't getting called.

Hardware wise:

Connection wise RX TX lines have been verified and its fine.

Could you please let me know what is getting missed here.

Is this to do with the PRIORITIES

I have changed the priority as well.

But it didn't work as expected.

Any suggestions would be useful

Thanks in Advance 

Sarjoon.