Checkout Process Skips Shipping Information

When checking out, the shipping page is skipped and it brings me directly to the payments page, which is step 2 of the checkout process. I had a program developed restricting the combination of products from multiple categories and had to turn of the combined shopping cart option in the modules/pages/login/header_php.php file. The error only seems to be occurring 1 out of every 4 checkout attempts. Is there more to turning off the combining of shopping carts, if so how am I able to do so? If this is not the problem what would be the causing this error?

I am using version: zen-cart-v1.3.8a

Thanks