I'm not sure if they have the $5 margin rule in here or not, but I think they might.
They do this on real margin accounts ... it may be $3 now ... used to not be able to short less than $5 stocks.
So if you own a stock that's worth less than $5 you can't use it's value for margin buys.
Even though the display shows you have it in buying power.
I had something like this happen to me once, but once I sold the stock that was around $1
I had full use of all my buying power back.