When I press "b" to set the altimeter at cruising altitude (which I have to do frequently because it keeps getting off), the autopilot does not correct my altitude even if I'm 1000 feet off. The only way to get it to climb or descend is to turn VNAV off and then turn it back on again. Now, if I turn off VNAV for the cruise phase and turn on ALT hold instead, everything works perfectly, but aren't you supposed to leave it on VNAV? I'm used to flying the 757 and 767 (Captain Sim), where VNAV worked much like ALT hold at cruising altitude. I'm sure I'm doing something wrong, but what is it?Thanks,boeing247