It seems that the issue might be related to how the transactions are being processed in sequence. If the first transaction executes successfully but the second one fails when run consecutively, it could suggest that the code does not correctly clear the current transaction in memory and get the next one.
See if any variables are storing details and not getting replaced with the next transaction details. Kindly debug and put a break point for the second transaction details. Analyze the variable values and see if there is any possibility for the above suggestion