Z32's have the same issue. If you do not get the RPM match just right then it does not like to go into first gear while rolling. I just blip the throttle (like heal and toe downshifting) to get the RPM's up and usually the gear shift will slide right in as the RPM's drop with just slight pressure. If it takes more than that I woulwd suggest bleeding your clutch and verifying the fluid level as well as checking the free play on your clutch pedal. As the parts wear on cars of this age it may need adjustment to properly disengage the clutch.