Yes we would have to be able to repeat it to fix it. Fixing web sites is like fixing cars. If you take your car to the shop and tell them it won't start, then when the mechanic turns the key it starts right up... they can't really fix it.
If you can't reproduce an issue on a web site and you do something to try to fix it... how would you test if your fix worked?
We first need to identify how to cause the problem. Maybe do more testing and try different things on the site until you get the problem to happen. Try using the same address and cart items from somebody that had the problem in the past.
Once I can see a problem, I can fix it... so work on that and post back here once you have steps to reproduce and I'll help you fix it from there.