I've made vases about 18" and 14" tall, not cone shaped though.
Sometimes as you pour the excess slip out it will 'glug' causing the soft slip to pull away from inside the mold. Be very careful and slow as you do the pouring.
Experience will show you how long to let the slip stay in the mold before you decant it. I found that the drier my mold, the less time I had to let it set up.
Perhaps one of your mold pieces has something on it that resists water, that can slow the movement of moisture into the plaster and cause thin/weak spots.
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