invalidate nstimer in objective-c iphone

I am having issue with invalidating a NSTimer. I initialize a NSTimer with below function.

-(void) initLTTimer{
    [self shutLTTimer];
    [self setQuestion:questionCounter];
    isQuestionAttempt=NO;
    tmLeftTime=[NSTimer scheduledTimerWithTimeInterval:1.0f target:self selector:@selector(updateLeftTime:) userInfo:nil repeats:YES];
    NSLog(@"Timer Initialized");
}

further I called a function updateLeftTime in its selector.

- (void)updateLeftTime:(NSTimer *)theTimer {
    NSLog(@"%d",timeCounter);
    timeCounter+=1;
    tfLeftTime.text=[NSString stringWithFormat:@"%d", (QUESTION_TIME_LIMIT-timeCounter)];
    if (timeCounter>=QUESTION_TIME_LIMIT) {
        if (isQuestionAttempt==NO) {
            [self increaseDropShots];
        }
        [self setQuestionBg];
        timeCounter=0;
        [self shutLTTimer];
        [self updateQuestion:nil];
    }

}

This function [self increaseDropShots]; called in above function. here is the code of this function

-(void)increaseDropShots
{
    NSString *imgName = @"DactiveRetake";
    if (IS_IPAD) {
        imgName = [imgName stringByAppendingString:@"_ipad"];
    }
    wrongAttemp+=1;
    for (int i =1; i<=wrongAttemp; i++ ) 
    {
        UIImageView *img=(UIImageView *)[self.view viewWithTag:i+50];
        [img setImage:[UIImage imageNamed:imgName]];
    }
    NSLog(@"Question dropped counter: %d",wrongAttemp);
    if (wrongAttemp == 3)
    {
        [self shutLTTimer];
        [CommonFunctions initGlobalVars];
        [Bgplayer stop];
        [Bgplayer release];

        OverPageViewController *opvc=[[OverPageViewController alloc] initWithNibName:[CommonFunctions getXIBFile:@"OverPageViewController"] bundle:[NSBundle mainBundle]];
        [self.navigationController pushViewController:opvc animated:YES];
        [opvc release];

    }

}

In this function I am killing this timer but I am unable.

   -(void) shutLTTimer{
    if ([tmLeftTime isValid]) {
        [tmLeftTime inva开发者_开发百科lidate];
        tmLeftTime=nil;
    }

} This is the whole scenario of my application

Please help me what is the issue.

Answered after conversation in comments

Does the code definitely get to the line, [tmLeftTime invalidate];? You can check this by using a breakpoint or an NSLog.