Zencart losing or dropping orders?

Let me preface this by saying I really don't think this is a problem with Zencart but I'm posting it just in case.

I recently replaced a customer's oscommerce store with Zencart. His sales immediately dropped by 40-50% and he is certain zencart is dropping the sales somehow. The checkout procedure of the two stores is virtually identical so I doubt it is a user issue and he hasn't been getting complaints from people not receiving orders.

1. Have there been any reports of orders failing to complete?

2. Any suggestion on some metrics or testing I could implement to prove this one way or the other?

Now reports of orders being dropped that I am aware of.

What is his traffic looking like? Is the traffic down, or is this a conversion rate issue?

I'd recommend installing the Google Analytics add-in to track what's happening on the site. There's also a user tracking mod which I believe will allow you track prgress around the site and hopefully identify any glitches.