[MAGENTO 1.9] Google Analytics Conversion Tracking not accurate

We need someone who is an absolute expert and knows what he is doing. The conversion tracking on Google Analytics is inaccurate every day. A usual day would be: tracked in GA: 15 orders Actual order in magento: 23 orders I can see the 15 order numbers in GA, but I cant understand why there 8 orders missing. What did we try: - Checked if it's a certain device/browser/mobile - cant see any pattern - Checked if it's a certain checkout way - cant find any pattern - Placed a lot of test orders - most work, some not - Checked and adjusted GA code on success page - All looks okay - Setup conversion tracking via tag manager - no changes - Cleared cache, CDN - no change - restarted ngnix - no change - compared time zone on GA and magento - both a same and correct Now we are at a point where we need some help from an expert. You need to analyze the situation, run some test and find the problem. What we will give you: - access to GA, magento, sFTP - coupon code to place test orders - our help whenever needed When is the project completed: Once GA tracking is 95% accurate over a 5 day period. Working duration: Work can be done only via US night time between 11pm and 8am (GMT-7)