First off, Pionus are rather famous for not wanting to come OUT, so that's a great problem to have!
-always reward. Age entrance with a treat. An almond is great because it requires them to sit a minute and eat it (thus having a few good minutes in the cage). I suggest having a certain treat that she ONLY gets when she goes in the cage.
-make sure the cage is a pleasant place with plenty of toys, enough space, and located in a place where she won't feel alone.
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