There was a thread on this subject in the late '90s, centering on Tru-Oil, which I think is a similar product. Several had experienced problems with slow, or non-drying while others had never experienced such problems.
The conclusion was that relatively long storage in partially-filled bottles was the culprit. A thick (but not hard) membrane of mostly-dried oil forms at the oil-air interface, and the concensus was that its formation depletes the driers in the oil that remains below the membrane.
I've added marbles to Tru-Oil and squeezed the bottle slightly to almost totally exclude air before tightening the lid, and have had good results from oil years old.
