rosella seems sad

i have a pair of scarlet rosellas very pretty. have had them for a year now. they been in the same cage and they have a nest box but the female has never bother to get in it. has never layed eggs since i have had her even though they are a pair. the past few days my female has been very sad. doesnt move much, sometimes seats at the bottom of the cage. doesnt look injured but sometimes walks as if she was or it looks like she walks to one side. they are not tame but lately has not try to get away from me when i reach for her. i dont know what to do because i dont know what all this symptoms mean. i dont want her to die. i have had female parakeets that have gone thru the same and most of the time end up dying after a couple weeks of this behavior. can you tell me what i can do or what this can be and what should i do to avoid problems she may have??