The problem for aaaaa is it gets to this parse (a(a(a(a)a) and can't complete the (aXa) parse for the second a, so it backtracks to try the (a) parse (a(

The problem for aaaaa is it gets to this parse (a(a(a(a)a) and can't complete the (aXa) parse for the second a, so it backtracks to try the (a) parse (a(a) The issue is since the parse of the third a as (a(a)a) succeeded, it will never try the (a) parse for the third a.