The problem was, the counter was being incorrectly reset to zero.
The counter was being correctly reset to zero. The nonce was being incorrectly not set to non-zero. (In CTR mode, there is a 64-bit nonce which is different for each message and a 64-bit counter which starts at zero for each message and increments as you move through the message.)
The counter was being correctly reset to zero. The nonce was being incorrectly not set to non-zero. (In CTR mode, there is a 64-bit nonce which is different for each message and a 64-bit counter which starts at zero for each message and increments as you move through the message.)