On my bourbon barrel aged maple syrup I dip the caps in wax, down to the neck (facing upside down). I never got a bubble. Are you filling all the way up to the cork? if yes, likely it's just where the syrup contracted as it cooled. I wax after the syrup is cooled.
AQt first I heated the wax in a dedicated sauce pan, rather tedious. After 2 year doing that I bought an electric wax heater. It was actually sold for waxing for hair removal (for ladies private spots) but it is perfect for waxing the bottles. It uses a 75 watt heating element, I plug it in, turn it to high and do something else until it's ready, about 45-60 minutes. Once all of the wax is melted I turn it down about 1/3. I then dip each bottle, pull it out, slowly turn the bottle 2 or more times until the wax stops moving, I then dip it in cold water and stand it up. The cold water makes sure the wax doesn't run any. After all of the bottles in the first case are done, I put them back in the case and I usually do 2 or more cases at a sitting. When done, I shut the Melter off, refill the wax with wax beads and let it cool.
The unit I use was only $19.99 about 5 years ago.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.