Yeah, that guy was me, and trust me I applied plenty of kinetic energy and appealed to gravity, to no avail. That sucker was stuck in there good.
I have some cores that slide loosely enough in and out of the plastic, I have others where you can literally watch the core falling into the plastic in slow motion as air is slowly displaced, and I have others that don't slide in with only gravity.
The one that I inserted backwards was one of those "slow motion" jyarz I think, and really it was my fault not to think that literally pushing the glass in and forcing the air out wasn't a good idea. It ended up completely inserted so that I couldn't even grip the glass to pull it out, and shaking didn't help one bit.
@marduk here saved the day by suggesting placing the jyar in hot water to encourage the plastic to expand a little and loosen up. Works great! It's no fun accidentally changing a jyar from loyal friend to foe holding your herb hostage…