Being made to pay Class 3 NI contributions from abroad

To Whom It May Concern, I live and work in China and will do for the foreseeable future, more than likely until the day I retire. I am a British citizen who has 15 years of NI contributions on my record and was born, raised and schooled in the UK thus I am qualified to pay Class 2 credits as I worked immediately before leaving the UK to live abroad in 2013, however, and this has been going on for two years now, the HMRC keep insisting that I am not eligible to pay class 2 credits even though I fulfil all the criteria, can anyone ask what would disqualify me from being able to pay class 2 credits when I am eligible (according to the wording of the eligibility criteria)? It doesn't say there is a qualifying period of how much work you had to have done before leaving the UK, just that you have had to have worked immediately before leaving the UK. All I get from them is 'you haven't paid enough credits' though I have actually paid 15 years, no-one has explained why I haven't paid enough credits?

Hi Carlos Hughes,
To be entitled to pay voluntary Class 2 National Insurance contributions whilst overseas, you must have lived in the UK for at least 3 continuous years since birth or have paid National Insurance contributions for at least 3 qualifying tax years.
In addition to this, you must have been normally employed or self-employed in the UK and are employed or self-employed overseas.
If you feel that you do meet this criteria then may we ask that you write to us and detail your UK and overseas employment again – such as if you were employed and the period involved (from … to …) and any periods of non-employment and the periods involved.
Our International Teams will be able to look at your application again.
Thank you.