Nara Shadow Jutsu

The nara clan jutsu are based off of stretching ones "OWN" shadow out to connect with anothers. They can only extend there shadow out from the shadows and cannot create shadows within another shadow.
SO for instance in shikamaru's chunnin fight with temari he stayed within a large shadow and extended out from there, as it became later and the sun set the shadow grew bigger and extends the range but if the shadow were to become to big and encompass the whole arena he would not have a shadow to extend from. Since shadows do not appear within another shadow his jutsu become useless.

This also explains in "The Inherators of the Will of Fire" why when shikamaru stops naruto from chasing kakashi that a cloud blocks the moon and releases his jutsu.

So with these simple and understandable facts about the jutsu we can determine the Nara clan gets stronger inbetween dawn and dusk. At night they are weaker because the shadows are less dense or apparent which ever way you want to look at it. Mid-day they are at there average level because they must strech shadows to go across much larger distances before connecting to another.

Shikamaru's range extends when he has a larger shadow to stretch BUT it can only strech the shadow outwards and not within itself.
SHADOWS CANNOT BE CAST WITHIN SHADOWS!!




Using other jutsu...
Nara members usually maintain a seal to keep there jutsu active, this is not the case with the regular shadow imitation jutsu. So other jutsu can be used while maintain the imitation technique. Since it is imitation the other person mimics all your actions but you do not control there chakra so they cannot use the same jutsu as you. If they have jutsu that require no seal you cannot prevent them from using it since you do not restric chakra flow.This also relates to use of equipment, typical nara have there equipment in different areas then a regular shinobi so that they can throw kunai or other things without risk of it being thrown back.