More information would be helpful but for starters please consider the following:
As your listed zone 4 area indicates, you have an extended storage period; this requires more careful storage conditions and requires strong healthy rhizomes to tolerate the long winter. You do not indicate at what temperature/humidity the rhizomes where stored. I personally prefer to use coarse peat for storage but sphagnum is fine. You do need to keep the filler slightly moist throughout the storage season. If it is allowed to become dry it will actually pull moisture from the rhizome. Preventing excessive air flow will aid in reducing drying. Do not use a sealed container.
Container grown cannas typically become overcrowded and the late season rhizome development is starved for nutrients and unable to store enough food.
Cannas that have or develop disease will be difficult to store. Canna virus along with fungal or bacterial infection can seriously contribute to storage problems. Bengal Tiger (Pretoria) is far from the most difficult cultivar to store but it is always easy.
Best,
Kent

Sorry for the last sentence. It should have ended with: if is NOT always easy. Kent