Worldpay integration problem to fix

We have a problem that since the Three website shops were integrated with Worldpay our sales have dropped a lot. This is what my website designer has to say:-

"I am pretty sure it's because of the WorldPay integration. Since its showing error to me, it might be showing errors to most of your customers too. I will suggest you to look for a developer who is experienced with WorldPay and PHP integration. It will be an hour's work for them. Since I do not have much experience working with WorldPay gateway, I don't want to break something that will cause you more problems"

Please be aware that 3 website shops are integrated to the one Worldpay Gateway.