My 2007 750 BF SOMETIMES takes extra long time to switch to 4 wheel drive,it has less than 400 miles on it....When I first got it switching to 4x4 or back to 2x4 took just a couple feet of movement,,yesterday in the slushy snow it was taking much longer and I could hear it trying to switch but didn't until after approximately 30-40 ft.......is this normal in those conditions??

 

Are you letting off the throttle while it try's to switch. Under load conditions it will not shift, the slush might be keeping it under partial load load making it take longer to shift.