I have also experienced this problem. I can be stopped at a light for even up to 30 seconds with the car in neutral (not moving or rolling forward at all). Put the clutch in go for first and it doesn't let you in. Its mechanically locked out, or so it feels. It seems to happen more in the cold when the engine/trani is not warmed up.
I have found out through experimentation though that it seems to be something to do with the reverse lockout. Maybe others can try this theory and report back. Basically the easiest thing is just pull up the reverse lockout when you go for first and in my car anyway it NEVER has an issue going into first, it goes in smooth as butter.
Also another thing to clarify my theory. If you go into the first gear position, pull up the reverse lockout, move your shifter left and let go of the reverse lockout it stays up. When you move the stick back to the right it pops down. This to me would indicate that the reverse lockout is blocking part of the first gear slot. This also seems to explain why its worse when the trani is cold, you are limited to the tolerance you have to get into the first gear position and the trani is not as forgiving when cold.
Hopefully some other folks with this problem (or pretty much anyone with a manual trani) can see if they also have a limited first gear position.