Both refer to failure, but
only ineffectual refers to the kind of failure that happens when
the effort was weak, impotent, and/or incompetent without
satisfactory or decisive effect. An ineffectual person does not
have the ability or confidence to do something well.
Ineffective means not
producing intended results and there is no effect. For example,
ineffective communication includes talking instead of listening
actively.
Likely ten percent
of people will notice or care when one of these words is used
instead of the other.