thanks for sharing your experience, and, fine that you were able to solve the issue.
Perhaps another thought about possible issues touching butterfly-C. Therein are found the differences between theory and practice :-)
Some (more) voltage at the driving shaft (middle of the C) may occur, if
- the loop is grounded, but not right to the electrical middle of the loop...
(The parts of bonding the cap to the loop prolongs the loop physically. If these connections are of different length at both sides,
this will shift the electrical ground-(neutral)point of the loop respective. Think about: Grounding ever happens if using a gamma match to drive the loop!)
- the two halves of the butterfly are not of equal capitance.
(If plates are of different aligned distances at both sides or plates are not of equal size. Just if homebrew, this may occur often.)