productsRequest response method is not calling

I am implementing an in app purchase and I am sending the request to apple store through

- (void) requestProductData
{
 SKProductsRequest *request= [[SKProductsRequest alloc] initWithProductIdentifiers:
         [NSSet setWithObjects: featureAId,featureBId,nil]]; // add any other product here
 request.delegate = self;
 [request start];
}

the response method

- (void)productsRequest:(SKProductsRequest *)request didReceiveResponse:(SKProductsResponse *)response
{
 [purchasableObjects addObjectsFromArray:response.products];
}

is not getting called at all. Only once did it call out of ten attempts I tried.