I think it is due to limitation in transaction size. Most likely you were mining hush for a longer time and filled your address in a lot of transactions. For example if you received 0.1 Hush in 1000 transactions, to send those 100 Hushes further, your transaction would have to contain information about all 1000 inputs, so most likely it will be oversized. You can join your many inputs by making smaller transactions to yourself and then you will be able to send all Hushes in one transaction.