The access to the tail is taken from the inside of the pony.
They /are/ hollow.
Have you left a bit of the wing plug behind?
Here's an old prepped Cadance (same mold as Luna)
Please ignore the butchered head plug. This was my very first princess bait and I didn't know you accessed the tail through the back.
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loginAccess to her tail is at the back, i just had to fish about with my hemostats to grab her washer.