28% Withholding tax for non US citizens/residents??

I realized 28% of gross precceeds is withheld by my trading agency and I wonder how I can get my funds back? I had no problems with this 28% for my previous trades before my w-8ben forms expired. However, every single transaction, regardless of its capital loss or gain, got taxed at 28% on gross precceeds and 30% on dividends. I had no problem with the withholding taxes on dividends, but my main concern is on the 28% on the gross precceeds. My trading agency said the 28/30% withholding taxes will still be in tact even after I submitted my w-8ben form. Is there anyway I could get my money back?( the money was a huge lum sum and I don't understand why they withhold so much even if i didn't make any profits or even lost money on the transaction.)
please advise.

I could tell you for sure, but I reckon you will need to submit a tax return