is it difficult to shift if you are stationary foot on clutch with the engine not running, if so I would say linkage and or selector adjustment. if it is only when you are driving is just the syncro working to allow the shift and thats fairy normal, and its harder to shift down at higher speeds, ps you should avoid shifting from 4th to 2nd as you have too great a shaft speed variation for the syncro, 4 to 3 then 2nd. avoid going back to 1st until you are almost stopped