I cant get into first gear?

Hi
My nissan micra, R reg, works perfectly well until ive been running it for about half an hour, when my first gear starts to stick.
It seems to go into first gear fine when i stop and take it out of gear then a couple of seconds later put it into first gear , for example at traffic lights but its more of a problem at roundabouts when i slow too much for 2nd gear and need to take off quickly it takes a couple of attempts to get it in.
I dont have a problem with any other gear at all ..any ideas?
Thanks