Poldark Masterpiece Theatre Seasons 3 and 4

On the purchase menue for Poldark Masterpiece Theatre (not to be confused with just plain "Poldark" which is the 1976 version) we have already purcased seasons 1 and 2. Now, only seasons 1, 2 and 5 are available. Seasons 3 and 4 are not available for purchase. Why is that and how do we purchase seasons 3 and 4 ?

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