NSUrlConnection hangs on didReceiveData, but not always - iphone

Question

I have a issue while downloading an image from the web.
I have a uitableview with fullscreen cells, every cell background is an image downloaded from the web using an array of urls.
The problem occurs only on the FIRST cell for the first time where the image doesnt load, but if I scroll down to the second cell and then go scroll up again to the first it loads successfully!
The app stucks on this method :
-(void) connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
NSLog(#"receiving data");
[downloadData appendData:data];
}
I have an iPhone 4s with iOS6 and this problem doesnt occur and the image loads perfectly.
Any suggestions?
Thanks in advance
EDIT
with my UIImageViewDownload.m that extends UIImageView
- (void) loadImage{
NSURLRequest *request= [NSURLRequest requestWithURL:myurl];
NSURLConnection *conn= [NSURLConnection connectionWithRequest:request delegate:self];
if(conn){
NSLog(#"Sto scaricando");
downloadData=[[NSMutableData alloc]init];
indicator= [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
[indicator setFrame:CGRectMake(0, 0, 30, 30)];
[indicator setCenter:CGPointMake(self.frame.size.width/2, self.frame.size.height/2)];
[self addSubview:indicator];
[indicator startAnimating];
}
-(void) connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
NSLog(#"ricevo data");
[downloadData appendData:data];
}
-(void) connectionDidFinishLoading:(NSURLConnection *)connection{
NSLog(#"finisco data");
UIImage *img= [UIImage imageWithData:downloadData];
[self setImage:img];
[indicator stopAnimating];
}