The old plaster is

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very dry.Use a brush or sponge and moisten the plaster with clean water.Moisten, don't soak it.

Put some water on it,the length of the joint,let it soak in then do it again.You don't want it wet,just moist.

The old dry plaster is sucking the water out of the drywall compound,and it can't cure right.Luck