Generate SO from quote - unit price greyed out

There might some rules applicable to the price that disallow a change because of the priced determination method or the fact the price came from a quote! Other than that, start by verifying the line is open and unshipped, the Allow Price Override option is selected in Maintain Items and you need to have the security right in SO to Change Unit Price in your security group(s).

This is working as designed.  The unit price cannot be modified on an order generated from a quote.  The logic is that it invalidates the quote given to the customer.