whats her excuse first off? if its something important like exams, obviously she cant come out
if she has no excuse, then theres two view points
shes a friend whos just busy/lazy hence you get to see her less often then you'd like, in this instance you simply arrange things with her but know it may not happen but make other plans with other people (i.e. dont depend on her for a social life)
or, shes a "sometimes friend" shes friends with you when it suits her, she makes plans with you as a back up, only to cancel them if something better comes up. in this case, dont bother, unless you have a lack of friends you dont need bad ones, if she wants to meet up with you, yea fair do's. just dont put her top of the list when your inviting friends out
hope this helps