And I had the same from my customer development team. The first thing I did was go to “Rollup Field” documentation in Microsoft Docs and sent to cross check the below points. After creating calculated fields over the years, I thought this has to be something related to configuration.
(Pic courtesy: Microsoft Docs)
Well, if you are wondering why your rollup field is not showing, most likely it is due to first of the two conditions mentioned.
Coming back to our scenario, client confirmed that they have not violated these scenarios. It was bit surprising as I had configured calculated fields in roll-up definition multiple times. So I had no other option but to open their calculated field definition.
And there I found it. What I realized is that beginners starting with roll-up field may make the same mistake too. Although their calculated field was not using other calculated field but they were using a field from parent entity in their calculated field definition.
If I take the second statement –“A rollup can’t reference a calculated field that uses another calculated field, even if all the fields of the other calculated field are on the current entity”, one of the implicit perception from this would be – “A rollup field can’t reference a calculated field that uses field of the parent entity in it’s definition”.
For experienced consultants, this may come in naturally. But what I realized from my experience is for beginners in Dynamics, they may tend to miss this point. The developer here just spent couple of hours trying to figure this out 
Hope this helps!
Cheers!
Debajit Dutta
(Dynamics MVP)
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