upload chunks using dropbox api

I am using the dropbox API to upload files in chunk. But the following code gets stuck at uploader.upload_chunked(). What might be the reason?

Also, is there any way to know what is happening in the background like uploaded these many chunks, still uploading, time taken to receive the uploaded chunks and other information ? Can threads be used to upload these chunks,assuming that the API takes measure to assemble the chunks in correct order,or is it that we have to take care of ordered assembling ?

import dropbox
size = 941   
access_token = 'xxx'
client = dropbox.client.DropboxClient(access_token)
to_upload_file = open('dummy.txt','r')

uploader = client.get_chunked_uploader(to_upload_file,size)

print 'uploading ', size
while uploader.offset < size :
        try:
                upload = uploader.upload_chunked(chunk_size=100)  
                print '100 bytes of chunk sent.'
        except dropbox.rest.ErrorResponse,e:
                print 'something went wrong'      
uploader.finish('/dummy.txt')

Edit:
size = 941 -> The size of the file to upload
upload=uploader.upload_chunked(chunk_size=100) -> chunk_size changed from default to 100 bytes
print'100 bytes of chunk sent' -> print statement included