Boeing 737 trim on takeoff

I'm revisiting FS after a longish lay off and vaguely remember having this problem before, but can't find the answer from searching the forum.If I create a flight with the default 737 starting at a gate and taxi to the runway using ATC etc, when I try and take off the trim seems to have gone very strange so that I can only actually take off by changing the trim appropriately at a speed way above V1/V2 or Vr.Is this something that rings a bell with anyone or am I being thick and missing something obvious??TIA for any help.Rob