I actually had mine stamped a few times as initially, I didn't know about it having to be done within 28 days of the date of the application, then it just took me a long time to get our application together!
I found that the result depended on the particular person that I saw.
The first time the girl at the desk just stamped and signed, no problem.
The next time, I saw someone more senior who asked a lot more questions but I got it done eventually.
Go back again, statements in hand, straight up to the counter and just ask them to stamp them as if it's something that's done every day. If that doesn't work, try another branch, and another.
I've seen reports of people having difficulty initially but I believe they've always managed with a bit of persistence.
Incidentally, if it's the signature that's putting them off, that's OK, just the stamp, on every page is enough.
Meanwhile, get them to mail the paper statements to you anyway as a fall back.
Who are you with?