Orders not being saved - intermittently

I have had my Zen Shopping Cart (Zen Cart 1.3.6) operating for approx 16 months now and have also had intermittent problems with Orders not recording. I love Zen Cart and I find it very user friendly but this is the one problem that is of major concern and it is frustrating me. I feel sure that my problem cannot be PayPal related because I offer a direct Bank Deposit payment option as well as PayPal - and have had the same problem with both orders paid directly into my Bank Account as well as Orders through PayPal. With the Direct Bank Deposit Orders I only know that I have an order when I see the Deposit in my Bank Account. I too have had to approach customers and ask them what they have purchased. It is very embarrasing having to go down that path and this week I had a large order cancelled (it was a pending ECheque done through PayPal) when the customer became concerned that there was a security problem with my Shopping Cart! The problem only seems to happen with large orders - not necessarily large orders with multiple items (although there have been one or two of these) but larger amounts- $1000+ Amongst many other items, I also sell Kilns and the ordering problem has occurred more often when I sell Kilns. I have always felt that it could perhaps be a weight related shipping problem although I had one large $ order (multiple items) that did not record and there was no kiln included? Another problem is that I did not configure or set up my own Zen Cart. I paid someone to do it and he is now living on the other side of the world! I have had this problem from the beginning. I think he may have upgraded to a newer version of Zen Cart to try and fix the problem but that did not resolve the problem. I purchased the Cucumber PDF hoping that I could find an answer but I may be looking in the wrong area/s for an answer? I would appreciate any advice or guidance very much.

Can you trigger the problem on-demand?, ie: place an order for $1,000+ using your Bank Deposit module, and see what happens? Anything unexpected? Anything different from doing an order under $1,000?
If you turn on the "Money Order" payment module temporarily and use that for a $1000+ checkout as a test (and Remove it again when done), how does that checkout process differ from the other one?

I recommend installing the Debug Logging utility first in case it can catch any system errors that might be happening -- see the link in this FAQ: https://www.zen-cart.com/tutorials/index.php?article=82. It's usually safe to leave installed long-term.

I think one site I deal with has had a similar problem. THis appears to be a result of a customer not fully completing the checkout procedure. In short, it appears they quit at the completion of stage 3 when making the payment, possibly believing that is the end of the process. Therefore, they never complete stage 4 when they would get the message "Thank you for your business" and when Zencart generates the success emails and store notification.

This gives the situation whereby the customer has paid but the store doesn't know about the order. We find out because our payment gateway emails us about a successful payment and is setup to show what has been ordered.

This problem has just popped up on my install 1.3.8 https://www.green-star.com.au .

Two eCheques have been received by paypal but no orders exist in zencart....