@ya_maryia - Without seeing your implementation, it's hard to know for sure, but there could be a variety of issues. I would start with the following:
Your "checkout" event could be firing less frequently than expected. Has your customer experience changed in the past few months such that a customer could bypass the checkout step? Are there errors preventing the analytics beacon from firing during checkout?
You are not passing a unique order ID (s.purchaseID) with every successful purchase and the "purchase" event is firing more frequently than expected. Are customers able to get back to the order confirmation page after the purchase (ie// page refresh, link in order confirmation email, etc)? If so, does the "purchase" event fire again? Is a purchaseID present on the call and does it change or stay the same?