Tail Chaining PendSV - arm

Question

I am creating an RTOS kernel using MSP432 Arm controller. I am using PendSV for context switching.
The issue is that when the systick handler sets PendSV handler but it is never called. I have the Systick handler at priority 0 and PendSV at priority 15. I am not sure what is going wrong but I have tried to minimize the program but this has not help. Any advice would help, thank you.
void SysTick_Handler(void) {
OS_tick();
__disable_interrupts(); // #suppress("Function cannot be resolved")
OS_sched();
__enable_interrupts(); // #suppress("Function cannot be resolved")
}
void OS_init(void *stkSto, uint32_t stkSize){
// set the penSV interrupt priority to the lowest level
NVIC_SetPriority( PendSV_IRQn, 0xFF) ; // #suppress("Invalid arguments")
OSThread_start(&idleThread,
&main_idleThread,
stkSto,stkSize);
}
void OS_sched(void){
if(OS_readySet == 0U){ // idle condition
OS_currIdx = 0U; // set oscurridc to idle thread
}
else{
// if else we have threads to run need ot do in round robin
do{
++OS_currIdx;
if(OS_currIdx == OS_ThreadNum){
OS_currIdx = 1U;
}
} while((OS_readySet & (1U << (OS_currIdx - 1U))) == 0U);
}
OS_next = OS_thread[OS_currIdx];
if(OS_next != OS_curr){
// Pend a PendSV exception using by writing 1 to PENDSVSET at bit 28
*(uint32_t volatile *)0xE000ED04 = (0x1 << 28);
}
}
void PendSV_Handler(void)
{
PendSV_HandlerAsm();
}