Baked sweet potato french fries (chips) too soft, how to crispen?
You can try a few things:


dredge the fries in cornstarch (whack them in a plastic bag, give it a good shake), shake off the excess, then season.
ensure the fries are in a single layer in the pan - crowding and stacking will mean they steam, not roast
when they're nearly done, turn the oven off and leave them in there for 20 minutes or so before serving.


I would also avoid cutting them thick. They should be french fries, not steak fries.