How to deal with HR for GC application?

My boss told me that my company decided to sponsor my GC, but he also told me that I should be proactive to contact HR to push things going on. I contacted HR people, the guy said he would contact legal department and tell me what to do next. But after a week, nothing happened. My company is a big company. I can understand it will not go very quickly. My questions are:

1) Usually, is it possible for HR to slow down the process deliberately? If yes, what are the most common reasons for them to do so?

2) How often should I push them (I don't want to make them impatient, but I don't want them to slow me down neither)? Also, any suggestions for how to push them?

Thanks a lot for any comments/tips!