I open mine in my own private chain, and I submit the transaction at the same time. Some of the transactions do not trade hashing, and then the fair is lost. Why?
Without more details about the transactions or how are they generated is hard to say exactly what is happening. A possibility is some transactions have the same nonce.
Transaction have a nonce field that is the number of transactions from the account sending the transaction.
You can submit two or more transactions with the same nonce, but only one will be confirmed in the blockchain.