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There are a few basic things that you can do to try to resolve the issue. First, do a shifter zero alignment and make sure that the linkage is properly adjusted.
Next:
Is your clutch fully disengaging? If the clutch slave or master has some air in the line or the fluid level is low, your disk may not me fully separating from the fly wheel and that can hinder shifting. Normally you would see the problem in other gears but you know Murphy's law.
Most likely a bent fork or a bad slider or dog inside the gear. There have been a few people that have ventured inside the tranny but, it's expensive and time consuming. You might want to talk to Rodney (he is the tranny GOD). he may be able to shed some light on to what is happening.