How to restore database using old full backup and current log file
It was essential that your database was in the Full recovery model and no transaction log backups were created in the last 15 days. That provided that all transactions that happened after the full database backup were still in your LDF file. Otherwise, there would have been a gap between your LDF and full database backup transactions and you would not be able to recover everything successfully (maybe that was the case)

There's another tool that can be used in the same scenario to read the transactions from the LDF file and create a redo script for them - ApexSQL Log.