just now

[No Name Available]

It sounds like exigency, as in "I can't oblige or accommodate you (or fulfill
your request) at this exact moment, or immediately, for I'm doing something
or I'm about to do something that is more urgent (a pressing need)."
Whenever I hear the expression,  the speaker seems to want immediate
resolution to a problem (or, at least, the hearer construes the request as
urgent and, thus, responds with, "not just now" when he or she is unwilling
or unable to respond immediately.   "Not right now" seems less urgent and
appears to pertain to a broader range of time than does "not just now."
Whereas "not right now" seems to equate with  "not at the present time," "not
just now" equates with "not at the present moment."  I guess the biggest
difference is in scope and the choice is motivated by the hearer's quickj
assessment of the exigency of the request in terms of his or her own
situation or plans.
                    Pat