The "Advanced Animations" flag in the FSX graphic settings should be checked. The GSX installer obviously checks for it and enables it automatically, but if you uncheck it manually afterwards or you reset the FSX.CFG to default, it will default to disabled.
If this is not the case, be sure you don't try to use a too wide zoom settings and/or a too far away view position, because after a certain distance and zoom setting, FSX automatically disable all animations.