Randomly missing transactions in Universal Analytics

Hello, I use your plugin and some of the transactions on my website are not tracked. Research shows that there are no thank you pages for some transactions, but for some of them the thank you page was tracked in Google Analytics but it sent 0 transaction value.
Could you help me figure out what is the problem?

Notes:
– The issue doesn`t depend on the payment method, it occures in different cases
– I use standart thank you page url –
/checkout/order-received/11111/?key=wc_order_qqqqqqqqqqqqqq