Question about fixing footer drains with no access
I encountered a similar situation. After extensive research I found a drainage contractor in North Carolina who recommends digging up each plank as needed and digging a ditch for channel drain. It looks like their deck in video below had wider planks so easier to dig in that slot after pulling off wood plank. Hopefully yours is wide enough.
FRENCH DRAIN UNDER WOOD DECK, Apple Drains, Charlotte North Carolina
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https://youtu.be/Z-mAzGY8-Zw