I observed that HR only follows standard formats of employment certificates/ letters. They become hesitant to entertain any request from employees when a different format of the letter is requested. Probably, because they are unaware of the purpose of your request. But that is just my thought... so this is how I approached it.
I prepared a draft of the letter. I talked to my immediate supervisor and asked him to confirm that the contents of the letter was accurate. I explained to him that I will be needing it for an application (to immigrate to Canada). He understood and he forwarded my draft to the HR recommending for it to be signed. I am very thankful that I have an open-minded Manager who supported me with my personal goals.
It was difficult requesting for it directly from HR. But when I asked my direct manager to assist me, it became easy. HR just needs someone to assure them that there is nothing wrong with the requested letter. Again,... this is just my perception. I am not saying that this is REALLY HR behavior.
Maybe you can try a similar approach. Look for someone who can persuade HR to prepare the requested letter.