Problem using convoy.

hi all

i ma using a convoy on my message so that all messages are started sequentially. for this i made a convoy on 2 elements ofthe message. everything goes fine unless the no of messages are less than 15 but as soon i send more messages in single batch it somehow starts picking infinite messages from msgbox. 

eg:

if no of messages in one batch was 25,, i  expect 25 responces but i get infinte messages ,, but if this batch size is reduced to 15 or less it works fine.

i m using 1 min delay while one instance of orchestration waits for the new message

can you guide whats the problem ??