I’m not sure why I can not trigger SVC exception second time in my program.

Here is my SVC hander:

void default_svc_call_handler(void){ 
__asm(
  ".global SVC_Handler_Mainn"
  "TST lr, #4n"
  "ITE EQn"
  "MRSEQ r0, MSPn"
  "MRSNE r0, PSPn"
  "BL SVC_Handler_Mainn"
);  
}

 __attribute__((used, noinline)) void SVC_Handler_Main( unsigned int *svc_args )
 {    
   unsigned int svc_number;
      
   /* 
   * Stack contains:
   * r0, r1, r2, r3, r12, r14, the return address and xPSR
   * First argument (r0) is svc_args[0]
   */ 
      
   svc_number = ( ( char * )svc_args[ 6 ] )[ -2 ] ;
      
   switch( svc_number )
   {  
     case 0:  /* EnablePrivilegedMode */
       __set_CONTROL( __get_CONTROL() & ~0x1 ) ;
       break;
     case 1:  /* DisablePrivilegedMode */
       __set_CONTROL( __get_CONTROL() |  0x1 ) ;
       break;
     default:    /* unknown SVC */
       break;
   }  
      
 }    

I used __asm("SVC #0); for the 1st call (SVC handler is triggered). However, I can’t trigger SVC second time.

How can I make sure the SVC handler is triggered?