Offering admission and then no response! I have applied in a US university (let say it is A) in a natural science department for PhD admission.
One weeks  ago I emailed the  graduate director to know my status because I received admission from another university. I wrote him that, I will not accept the other university's admission offer if I'm in their consideration pool. The graduate director emailed me back that I have received the admission. He also suggested me to sign the offer letter he sent me  and to complete the admission within two weeks. 

After carefully checking the email, I saw that  I didn't  get  the offer letter in the attached file. I replied him back immediately regarding the missing attachment. I replied the professor two times regarding the missing attachment  in the consequent  two days but still I haven't received any reply. The admin and the graduate director are not responding whereas 7 days have been passed. 

Somehow I had a connection with a current student who is studying (PhD) in that department and I requested  him  to talk with the admin.  He discussed my matter with the  admin who writes and handles the  documents. The admin told him that, She didn't process any document for me regarding admission offer.

What should I do in this case? I have another admission offer from another university but I'm very much interested to the program I'm taking about. 

EDIT: I'm an International Student
I expect by now you have had to decide - but

Often there is only one faculty member (grad advisor) responsible for day-to-day worrying about graduate admissions in a medium size department in the US. And it is only one of his/her responsibilities. The best assumptions are that email went astray or buried (make sure to check your spam filtered email also!), or he/she got distracted by other deadlines and cannot get back to it for a few days.

As you did, polite emails (not frantic or demanding) to request clarification are the way to go. Recognize that if there is a large time difference, that it will create slow motion exchanges. A distraction for the faculty member may put the response 2-3 days later. (Make it easy to find the email if it was buried after a day or two by putting your name and some concise note or something like 'grad admissions clarification question" in the subject line 

Pay particular attention to the first email you received that said you were admitted but neglected to attach the form you expected. Were there several names cc'd on the email? Make sure to keep those people cc'd on your requests to keep them in the loop. It may be that they can pick up where the first person left off.

And finally, if you decide to accept another institutions offer, then after you have settled that, then send a polite letter to inform the department. It may be that you decide later to switch schools and want to try again with this department.