Jira return null values for issues

[Volen Pavlov]

Hello,

Well, my question is more theoretical as I do not have code to show and possibly look for the mistakes. 
To explain a bit. We have set up ETL to get all the data we have on JIRA(about 60k rows).
The problem is that when we run the ETL day by day there are lots of differences, last time it was in 10k records - this is very relevant to the dates based on the statuses in the history(in development date, under consideration date, ready to ship date, etc.)
Sometimes for some issues we get the correct dates, the next day they are "null", the day after they are again correct, but we are never able to capture the whole picture. 

Is it possibly because we make around 60k calls to the API for every single issue in the history?
Or maybe the problem is somewhere else.

One more question, can we make an export for 60k rows at once? If yes, how and if no, what is the alternative?

Thanks for you input!

Best,