Impression Share Not Coming Through On All Keywords, Why?

Not all the impression share data on all the keywords are displaying when I bring them through the API, shows Can't be calculated, however when I go into the AdWords interface its there.

 The funny is that it shows on some keywords and not on others, looking at one report, it shows on 200 keywords and not on 1. The 1 however is a popular keyword with high search volume. It is enabled.

I have run it multiple time every time the same applies.

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