Can't capture ppp packets using winpcap sometimes

I am using winpcap on my Windows XP, wanting to grab ppp packets with my WCDMA card. I have installed winpcap 4.1.3 and Microsoft Network Monitor 3.4. I am compiling the basic_dump example code from the winpcap dev kit. It can always display ppp map:

1. \Device\NPF_GenericDialupAdapter (Adapter for generic dialup and VPN capture)
2. \Device\NPF_{04F9E6B9-214E-4FE5-892B-0419694392E1} (WAN (PPP/SLIP) Interface)

      

        
        
        
      

    

Sometimes it grabs packets and prints the timestamp and length as expected, sometimes it just doesn't print anything even when I surf the internet. Below is the code.

#ifdef _MSC_VER
/*
 * we do not want the warnings about the old deprecated and unsecure CRT functions
 * since these examples can be compiled under *nix as well
 */
#define _CRT_SECURE_NO_WARNINGS
#endif

#include "pcap.h"

/* prototype of the packet handler */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data);

int main()
{
    pcap_if_t *alldevs;
    pcap_if_t *d;
    int inum;
    int i=0;
    pcap_t *adhandle;
    char errbuf[PCAP_ERRBUF_SIZE];

    /* Retrieve the device list */
    if(pcap_findalldevs(&alldevs, errbuf) == -1)
    {
        fprintf(stderr,"Error in pcap_findalldevs: %s\n", errbuf);
        exit(1);
    }

    /* Print the list */
    for(d=alldevs; d; d=d->next)
    {
        printf("%d. %s", ++i, d->name);
        if (d->description)
            printf(" (%s)\n", d->description);
        else
            printf(" (No description available)\n");
    }

    if(i==0)
    {
        printf("\nNo interfaces found! Make sure WinPcap is installed.\n");
        return -1;
    }

    printf("Enter the interface number (1-%d):",i);
    scanf("%d", &inum);

    if(inum < 1 || inum > i)
    {
        printf("\nInterface number out of range.\n");
        /* Free the device list */
        pcap_freealldevs(alldevs);
        return -1;
    }

    /* Jump to the selected adapter */
    for(d=alldevs, i=0; i< inum-1 ;d=d->next, i++);

    /* Open the device */
    /* Open the adapter */
    if ((adhandle= pcap_open_live(d->name,  // name of the device
                             65536,         // portion of the packet to capture. 
                                            // 65536 grants that the whole packet will be captured on all the MACs.
                             0,             // promiscuous mode (nonzero means promiscuous)
                             1000,          // read timeout
                             errbuf         // error buffer
                             )) == NULL)
    {
        fprintf(stderr,"\nUnable to open the adapter. %s is not supported by WinPcap\n", d->name);
        /* Free the device list */
        pcap_freealldevs(alldevs);
        return -1;
    }

    printf("\nlistening on %s...\n", d->description);

    /* At this point, we don't need any more the device list. Free it */
    pcap_freealldevs(alldevs);

    /* start the capture */
    pcap_loop(adhandle, 0, packet_handler, NULL);

    pcap_close(adhandle);
    return 0;
}


/* Callback function invoked by libpcap for every incoming packet */
void packet_handler(u_char *param, const struct pcap_pkthdr *header, const u_char *pkt_data)
{
    struct tm *ltime;
    char timestr[16];
    time_t local_tv_sec;

    /*
     * unused parameters
     */
    (VOID)(param);
    (VOID)(pkt_data);

    /* convert the timestamp to readable format */
    local_tv_sec = header->ts.tv_sec;
    ltime=localtime(&local_tv_sec);
    strftime( timestr, sizeof timestr, "%H:%M:%S", ltime);

    printf("%s,%.6d len:%d\n", timestr, header->ts.tv_usec, header->len);

}