Post grad is higher than a bachelor's, he applied for regular diploma which could be an issue but getting the notes is the best thing to do.I am a lawyer in Brazil. (Third world country)
I did not have IELTS score.
I applied for a post-graduate program (Paralegal).
And I received AIP.
So... you don’t need to apply for a Master.
Maybe your letter was not quite so good.
I said that the most important thing at this course was the network. As a lawyer, it would increase the numbers of clients that my office already has.
Ask for GCMS and try again.
Good luck.