Are you running any mods? Which version of Silent Hunter: Wolves of the Pacific do you have, and where did you purchase it?
First thing to check is your crew screen, and see if everyone has a little green bar next to their box, showing good health, especially for the engine room guys, but also the control room. Are all of your positions properly staffed? If you're missing people from the important locations (Repair Crew can stay empty), you'll have to fill the vacancies before going out. You can also try and see if they can comply with a Reverse Speed request, or a rudder change. Also, look in the upper right corner of your screen, and make sure you aren't at battle stations. Had you done any <Alt><Tab> moves, or the <Windows> key to get to the desktop while in-game? That can cause issues. If you haven't left port yet, exit the game (don't save) and re-boot the computer. See what happens.
Dates and upgrade packs can cause issues, but that's usually if you arrive about the time a calendar month that has 30 days (or 28) would end prior to your departure. August 15th, you should have been fine... But, just to let you know, the game has September 31, June 31, February 29, etc., peppered throughout, and that does cause issues under certain circumstances. You might try to reload that last save you have again, only stay out a few weeks longer and don't come back until November 1, 1944. If you want to try doing a Status Report, you might get an assignment that could help pass the time. Just do a quick re-fit at a different base somewhere and see what you get. Do another save under a different name before you try to re-fit, just in case...