Receive Inconsistency in the dspic33f controller using Interrupts

Question

I m using dspic33f series micro controller to receive SMS from modem SIM800A. I m using interrupt method to receive response from the modem using serial communication . If I send more than one AT command to modem via UART sequentially I'm getting response into the recieve buffer only for 1st command and no response in the receive buffer for the remaining commands though the modem was responding. After debugging I found the reason for this peculiar behavior. The reason was I was clearing the receive buffer after receiving the response into the buffer and printing on the console i.e. before sending next command to modem via UART using memset function in c. But on commenting this memset function i was able receive the response into the receive buffer for the all the AT commands that was sent sequentially, if the memset function is not commented then no response is filled in the receive buffer though the modem was responding so please help out in receiving the response into the buffer.

Code which i have written

#include "p33FJ64GS606.h"

#include <stdio.h>
#define FCY         40000000UL
#include <libpic30.h>
#include <string.h>
_FOSC(FCKSM_CSECMD & OSCIOFNC_OFF)
_FWDT(FWDTEN_OFF)
_FPOR(FPWRT_PWR128 )
_FICD(ICS_PGD1 & JTAGEN_OFF) 

char StringLoop[161];
void Init_Uart1(int uart_no)
{   
    U1MODEbits.STSEL = 0; // 1-Stop bit
    U1MODEbits.PDSEL = 0; // No Parity, 8-Data bits
    U1MODEbits.ABAUD = 0; // Auto-Baud disabled
    U1MODEbits.BRGH = 0; // Standard-Speed mode
    U1BRG = UBRG1_VALUE; // Baud Rate setting for 115200
    U1STAbits.UTXISEL0 = 0; // Interrupt after one TX character is transmitted
    U1STAbits.UTXISEL1 = 0;
    U1STAbits.URXISEL0 = 0;
    U1STAbits.URXISEL1 = 0;//jkv
    IEC0bits.U1RXIE = 1;
    IEC0bits.U1TXIE = 0; // Enable UART TX interrupt
    U1MODEbits.UARTEN = 1; // Enable UART
    U1STAbits.UTXEN = 1; // Enable UART TX
    U1MODEbits.USIDL=0;
}

void UART1_puts(unsigned char data)
{   
    while (U1STAbits.TRMT==0);
      U1TXREG = data;


}

void UART1_send(unsigned char *s)
{
    memset(StringLoop,'\0',sizeof(StringLoop)); /* if I comment this line then i can receive the response into the buffer StringLoop continuously else i receive the response only for the 1st AT command and I don't get the response for other AT commands into the StringLoop though the modem is responding*/
    while(*s)
    {   
    UART1_puts(*s);
    s++;


    }
}

void __attribute__((interrupt, no_auto_psv)) _U1RXInterrupt(void)
{

    if(IFS0bits.U1RXIF)
    {

       StringLoop[rcindex++] = U1RXREG;
       if (rcindex >= (sizeof(StringLoop) - 1))
            rcindex = 0;          
  }  
    IFS0bits.U1RXIF=0;
    }
}
void main()
{
    int i=0,j=0;
    Init_Clocks();
    Init_Uart2(1);
    Init_Uart1(1);
    TRISFbits.TRISF1=0; 
    LATFbits.LATF1=1;
    UART1_send("AT+CMGR=1\r\n");    
    __delay_ms(2000);
    printf("stringloop is %s\n",StringLoop);

   UART1_send("AT+CPMS=?\r\n"); 
    __delay_ms(2000);
    printf("stringloop is %s\n",StringLoop);

    UART1_send("AT+CPMS?\r\n"); 
    __delay_ms(2000);
    printf("stringloop is %s\n",StringLoop);

}