objective c - NSOperation Queue executes only first operation and stop executing after that -

i trying execute custom operations in nsoperation queue, problem executes first operation in queue , not execute rest of them. please resolve issue.

here code:

for (contentmediamodel *uploadmedia in mediaarray) {     amazonupload *customoperation = [[amazonupload alloc]    initwithdata:uploadmedia];     [operationqueue addoperation:customoperation]; } 

here custom operation class:

 @implementation amazonupload  -(amazonupload *)initwithdata:(contentmediamodel *)uploadmedia { if (self = [super init])  {     self.uploadmedia=uploadmedia;     executing = yes;     finished = no; }   return self; }  -(void)start {  nslog(@"** operation starts **");  [self main]; }  -(void)main { if ([self iscancelled]) {     nslog(@"** operation cancelled **"); }  nslog(@"** operation executing **"); [[amazons3manager sharedinstance] uploadfile:self.uploadmedia.media.filename     showloader:no]; }  -(bool)isconcurrent {  return yes; }  -(bool)isexecuting {  return executing; }  -(bool)isfinished {  return finished; } @end