i had the same problems when trying to sell some of my pennystocks in 2012 so i transferred those that i knew would sell to a bank related broker (i emailed them before asking them about the tickers & the DTC fees) and there was no DTC BS commissions like my broker was tryiing to charge me the $450 i rather pay a higher commission than a $450 fee ..LOL !!!! but this was when PENSON was having more lawsuits beeing slapped against them and their stock was dropping, also all the new rules started surfacing by the Regulators.
but before having to transfer those Pennystocks the agent told me this below, i was like WTF ?
This will be down to a new rule deployed by the clearing corporation and the exchange themselves. (aka Penson)
You cannot trade more than 10% of the 20 day moving average in one order. There is also the additional requirement that your order must not be anymore than 20% of that particular days volume.
then i decided to do the transfer and switch to OPTIONS less BS, less Manipulation, no 3rd party Bozo's 
(besides the media and analysts BS)
hope this helps
$TYTN
