The rules for visas into China vary from country to country and they change constantly.  Right now, as I understand it, several nationalities (including Americans) are having difficulties getting F visas over 15 days.

General rule is to have invitation not only from local company, but also from certified institution of local government (ask company for help, they should know  right institutions and persons).

Main difference is those "extra" invitation".

Btw. it will be always easier if You have previous "business visa history".

Other requirements vary, depending on Your nationality.

Latest information are available in consular services of Chinese Embassy in Your homeland, but also Your embassy here should have latest news, confirmed, and much more detailed then those available on internet forums. 

In China, contrary to other countries, you are guilty until proven innocent, and this particularly applies to visas.  and today, those of us who are holders of Chinese visas, must pay for those smart asses who got whatever they could get, and once inside did as they pleased.

Please understand that in a visa petition you can request anything you want, but in order to get anything above the bare minimum, you must have a record built of respect and obedience to their laws and regulations.

I started visiting China around 1997, and since then I have received over 10 visas.  My first one was a single entry, 30 days, and I was given a 30 days extension, but just stayed 12 days. from there I was granted on validity a three months visa, then a 6 months, after that one year and now I do get two years. On the entries, as new visas were granted, I went from one entry to two, and finally got the "M" for multiple entries. And in days of stay, from 30 on the first one, I now get 120 days, and was told that next time will be 180 days.

But I do not renew my visa in Hong Kong, they are not as generous there as they are back home.  And I do submit all documentation requested, and a few not requested, and attaching letters of explanation as to why of some of my requests.

And another thought to keep in mind.   How does your home country treats Chinese applicants for visas ?  If nice, easy and cheap, you will get the same back, if not, you get the special treatment.