Problem in Tap Detection and its Callback with MPU9150

Hey all,
I am trying to use the tap detection feature of MPU9150. Here is the code which I used.

dmp_enable_feature(DMP_FEATURE_TAP);

dmp_register_tap_cb(tap_cb);

The callback function is not being called.
Also, In my research i found this function:

static int decode_gesture(unsigned char *gesture)

{

unsigned char tap, android_orient;



android_orient = gesture[3] & 0xC0;

tap = 0x3F & gesture[3];

if (gesture[1] & INT_SRC_TAP) {

.

.

.

}

Here I printed value of gesture[1], which remains 0 while stationary, then goes to 4 and is steady there. No more changes.

Can anyone tell me what I am missing? Is there any interrupt to be enabled?? Or any condition to be matched??

Thank you in advance.