Genuine cross bars don't fit onto roof rails?

Hi all, I just wanted to ask if anyone has any advice please; I have a FL2 with roof rails fitted, I'm now trying to fit the genuine cross bars which attach to the rails but they are too short to actually grab onto the rails? The rails were already on the car when I bought it so I'm unsure how to check if they are genuine or aftermarket rails (I just assumed they are aftermarket and now I'm thinking they must be as the genuine would definitely fit) so I wanted to ask here in the hope someone knows if the genuine cross bars are actually compatible with aftermarket rails or not before I list them for sale!

Thanks all I really appreciate any help at all!!

I believe there are 2 lengths of cross bars, one for vehicles with a sun roof, and a set without sun roof. The sun roof version of the rails are wider apart, to accommodate the sun roof itself.
Of course I could be talking rubbish, but I do seem to remember there is a difference between sun roof and non sun roof models.