why can utp_write only be called once in a callback function

[ac169]

why utp_write can only be called once in the callback function, if calling utp_write again to send data has no effect.

// Callback for when data is received
uint64 on_read(utp_callback_arguments *a) {
    printf("Client received %zu bytes: %.*s\n", a->len, (int) a->len, a->buf);
    fflush(stdout);
    return 0;
}

// Callback for when a connection is established
uint64 on_connect(utp_callback_arguments *a) {
    printf("Client: Connected\n");
    const char *message = "Hello from client!";
    utp_write(a->socket, (const byte *) message, strlen(message));
    fflush(stdout);
    return 0;
}

// Callback for sending data
uint64 on_sendto(utp_callback_arguments *a) {
    int sock = (int) (intptr_t) utp_get_userdata(a->context);
    ssize_t sent = sendto(sock, a->buf, a->len, 0, a->address, a->address_len);
    return (sent < 0) ? 1 : 0; // Return non-zero on error
}

...
    int sock = socket(AF_INET, SOCK_DGRAM, 0);
...
    utp_context *ctx = utp_init(2);
...
    utp_set_userdata(ctx, (void *) (intptr_t) sock);
    utp_set_callback(ctx, UTP_ON_READ, (void *) on_read);
    utp_set_callback(ctx, UTP_ON_CONNECT, (void *) on_connect);
    utp_set_callback(ctx, UTP_ON_ERROR, (void *) on_error);
    utp_set_callback(ctx, UTP_ON_STATE_CHANGE, (void *) on_state_change);
    utp_set_callback(ctx, UTP_SENDTO, (void *) on_sendto);

    utp_socket *s = utp_create_socket(ctx);

    utp_connect(s, (const struct sockaddr *) &addr, sizeof(addr));

    byte buffer[65536];
    while (1) {
        struct sockaddr_in src_addr;
        socklen_t addrlen = sizeof(src_addr);
        ssize_t len = recvfrom(sock, buffer, sizeof(buffer), 0, (struct sockaddr *) &src_addr, &addrlen);
        if (len < 0) {
            perror("recvfrom");
            break;
        }

        utp_process_udp(ctx, buffer, len, (struct sockaddr *) &src_addr, addrlen);
        utp_check_timeouts(ctx);
    }

    utp_destroy(ctx);
...