For some reason, Datsun gearboxes that have seen some [ab]use seem to be a little slow into second. Particularly when cold. My original 1000 was like it, the 5 speed in my '84 Bluebird, along with the 5 speed GX box in my 1000 were like it, & the '82 Bluebird TRX five speed in my 200B is still like it.

The fix? After following ddgonzals suggestions, try taking it a little more gently into second, & let it warm right up before pushing it.

Can't do this? Then a strip & overhaul would be the task of last resort, as the second gear synchro has possibly worn to the point of being out of service limits.