My IB account has a 50% maintenance margin. My share value has consistently been more than my cash value recently and it doesn't seem like IB charges me anything for it. I'm borrowing money from them essentially, shouldn't I be charged interest for it?
If the amounts are reasonably small, it takes them a while to accrue interest. But rest assured, the interest is accruing, and they will deduct it from your account, once a month, if you have borrowed money from them.