Issues with App Install Campaign performance


We’re looking to implement, using Ads API, video-based app install campaigns but before finalising we need to understand why we are getting such different unpredictable behaviour from seemingly identical campaigns , otherwise it will be very difficult to sell our Ads API-based app to prospective advertisers
For example, we ran 2 identical campaigns in November, 2 weeks apart, on our own @Hoprapp account , targeting iOS users (v8 and above) in California , with bidding optimised for installs, using the same 4 video-card tweets. The first campaign yielded more than 188K impressions and several installs with CPA well below our maximum bid, the second campaign 40K impressions and zero installs
We have gone through multiple iterations of this process now over a period of several months, with multiple conference calls with the UK Twitter business team but to no avail
Has anyone else cracked this issue?