It's hard to say what you should really do to 'fix' your problem, but let me go at it from a different angle.
Idealy, a home's humidity level should be set to between 35 +/- and 40 +/- percent humidity. (Higher-setting in Winter, lower-settin in Summer ...) With that, the floor will move but slightly. That shouldn't cause too much movement but maybe enough for a little character.
Now, IF the floor was installed when the humidity was high, beings that you managed to lower the humidity, the flooring has decided to 'shrink'. You have a few choices, which you will do based on the outcome you desire.
1) Leave the floor alone. You are willing to live w/the lower humidity AND the separation in the floor. 2) Re-install the floor now that the humidity is at a desired level, which will eliminate the gaps. 3) Leave the floor alone BUT remove the barrier so the flooring re-expands, and you'll be willing to live w/the floor and the house as it was originnally 'set'.
I don't suggest you use any rope or filler to fill the gaps UNLESS you've managed to stabilize the humidity levels in the house. I'd stay away from filler and consider rope. You can pre-stain it and it will 'move' a just enough to keep the gaps 'filled'. If the floor moves TOO much, no matter what you put in there, it will come out. Yes, roping is a 4th option but I would install a humidifier first, then let the floor go through 4 seasons, then fill w/rope (assuming you like this option.) Be sure to measure/monitor the gaps through the 4 seasons so you know what the floor is 'doing'.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!