Rejected for new phone contract (twice) !!!!

Afternoon!
I am done trying to buy a new phone via o2 now.
I actually did tried twice at the shop and then once via deinHandy but every time they rejected my request saying ("Due to your security reasons").

I googled around and found out that they check for "solvency" and all that which is fine by the way but still I am NOT SURE why they cancelled it.

I am a full time Student (it's been almost a year now) I work as HiWi too and I am very serious about both.

What more you need to see?

Best,
Mohan