Triumph Repair: clutch/transmission, triumph spitfire, double clutch

Question
I've a 1976 Triumph Spitfire which although shifts up through the gears smoothly, will not gear down from fourth to third.
It will gear up and down when not in motion.
Some suggest pressure plate others tell me its common and learn to double clutch.

Could you suggest what thing(s)might cause such a problem?

Thank you

Answer
Hi Robert,
If you mean that it grinds the gears when shifting down, then the syncros are either worn or you have oil that is too thick in the gearbox. Also the procedure you use to down shift can be a factor.
Read my tech tip on how to make a standard transmission last.
http://mg-tri-jag.net/tech4.htm
Howard