Today at 3:57pm, I tried to shortsell a 1,000 shares of XYZ at market. I was filled at 15.39. The other 500 was not filled. I asked the broker why because it was put in at market. They stated that there was no uptick between 3:57pm and 4pm. I thought that was strange because there were many trades at 15.40, 15.41 etc. As a matter of fact it closed at 15.40. So, I think that there must have been an uptick between 357 and 4pm. OR Am I misunderstanding the uptick rule?