From your latest information and description that it sent you to the future when lightning striked but weren't electrocuted I'm assuming you were some distance away from it when teleported. Meaning it sent ya into the future rather the past. You were in the periphery field you described. So to get back you'd have to be near the center of the field when another strike happens.
The obvious issue is isolating yourself from the lightning bolt. That should change the polarity you require. Slowed time fields teleport you to the future, strong ones into the past.
Be warned though, depending on the time of year or day you can overshoot your destination back. I would do it on the same day of year you left from at the same location if possible. Little difficult if you need a lightning strike. Could simulate the effect using a large bank of high voltage pulse capacitors. The same ones used for quarter shrinking.
Considering the machine wasn't send into the past but rather both of you into the future, I'd say the lightning strike didn't make a field strong enough to travel into the past. Either because the strike wasn't strong enough(unlikely) or most of the bolt got grounded another way. In that case you have to redirect the whole discharge into the machine. Might have to make it beefier to prevent damage but still not much should occur because you can Earth ground it.
Was your pottery shed made of metal or wood?
This 100year travel span range with a lightning strike is what I have heard happens from another source. Fascinating.
Here's another possibility, if the field diffuses it's time field too much it might only be a one way to the future time machine. In that case you'll have to find a way to concentrate the field instead of spreading out. That might also be the polarity issue you're having difficulty with.
A couple possibilities:
1)only partial lightning strike powered the machine
2)device is a one way to the future machine due to time field diffusion
Either need more power or a way to concentrate the field. A 3rd possibility is the reverse time travel phase conjugation effect is not occuring due to the torsion effect of the machine. That would also prevent backwards time travel. You'd have to find a way to relax the machine's torsional effect upon the aether. Then you'd travel back in time easily.
If I'm correct about the design you used it also generates a magnetic field so the field should be locally contained within it. So the problem is the torsional effect. Magnetic fields also produce a torsional stress on the aether preventing backwards time travel.
That's probably the issue.
Here's a possibility, if you added an attracting magnetic field underneath the first one then it could fix the problem. Relaxes the torsional effect in the center of the field. Would have to encompass the machine and yourself to do so.
Still depends on the exact design you use though. If you have detected signals from the future using it I'd guess you're using an inert gas, pure quartz glass lenses, or certain crystalline materials as well.