On the way down to Egypt Yaakov comes to Be'er Sheva and brings
korbanos "leilokei oviv Yitzchok" - to the G-d of his father Yitzchok.
(46:1) Rashi says that Yaakov mentioned only Yitzchok, and not Avrohom,
and from this we are to understand that a person has a greater obligation
to honor his father than his grandfather.
While this lesson is readily understood, it is difficult to understood
why previously Yaakov davened to the G-d of both Avrohom and Yitzchok (see
Parshas Vayishlach 32:10). If having one's father in mind more than one's
grandfather is an appropriate mindset when bringing a karbon or davening,
then why does Yaakov not do so always?
Yaakov's korbanos in Be'er Sheva are brought on the heels of hearing the
news that Yosef is still alive. Perhaps, at this juncture of feeling the
relief of Yosef's well-being and the renewed opportunity for Yosef to
practice kibud av in person, Yaakov was more sharply focused on the direct
father-son relationship and therefore had only Yitzchok in mind at this
time. (See also the many midrashim connecting Yaakov's obligation of kibud
av to Yitzchok with Yaakov's loss of Yosef's kibud av.)