Self employed but no option to pya voluntary class 2 NICs

I have been self employed, and filing a self assessment tax return for a number of years. Despite saying I want to, I have never been given the option to pay any class 2 NI - no matter what I say my profit has been. Now I am in the position to want to pay for missing years in my NI record but I cannot see how to do it. Future Pensions have confirmed that I can fill missing years. The only information I can find online is to register for self assessment - which I am already registered for.

Hi,
Please find guidance here:
Pay voluntary Class 2 National Insurance contributions if you do not pay through Self Assessment and,
Voluntary National Insurance.
Thank you.
 

Good luck! My wife has been struggling with this for years and has lots of missing years as a result. She used to pay voluntarily, but for some reason it's become impossible for her to do it. We don't know why.