For long-term storage we always transfer our cells to liquid nitrogen, preferably as soon as possible after freezing. This helps to preserve the cells better. Also, how were the cells frozen? Because this can also impact the amount of living cells after thawing.
We always store B16F10 in liquid N2. It's best for any length of time to move cells to colder storage if possible. We preserve our cells in DMEM 10% FBS 5% DMSO. It's also important to follow good practice when bringing cells out of storage, but if you know they had been stored at -80 for a long time then that's the most likely cause of cell death.