Query Binding with question mark does not work

I have been using the question mark all the time to mark the location of the bound values in my queries until I have met one short line of code where the question mark has refused work. I don't know why.
There notes at query bindings state that

Instead of using the question mark to mark the location of the bound values, you can name the bindings, allowing the keys of the values passed in to match placeholders in the query

I replaced the question mark with a named binding and the code worked! Why the question mark does not work, I cannot tell at all. I have tons and tons of code that use the question mark.

Can you show us the query code and the array being passed to it, so that we can help you?

What did you Try? What did you Get? What did you Expect?

Joined CodeIgniter Community 2009.  ( Skype: insitfx )

Can you show us the query code and the array being passed to it, so that we can help you?

As I stated, it was only a tiny location in my huge project and I fixed the problem by using name binding. The remaining code that uses the question mark is working without any problems.