Shulchan Aruch ( O.Ch 1: 23) “It is good to say פרשת המן.”
(Levush) So that you will believe, Parnasa is Hashgacha Peratis
The Aruch Hashulchan writes:
Our Minhag is not to say it
There is no mention of פרשת המן in Shas
The Perisha says there is a Yerushalmi (Brachos) that speaks
about the Segula of פרשת המן.” The Aruch Hashulchan writes
he couldn’t find this Yerushalmi. (It hasn't been found since)
He continues The reason for not saying פרשת המן is because:
a) “The Parsha mentions numerous times the Bnai Yisroel's
complaints of not having food.
b) The complaints on the מן
c) The Midrash writes Hashem said “I don’t need not your
complaints and not your praises." (on the מן)”
According to the Rambam*, If the sole purpose of saying the Parsha,is to have the Segulah of Parnasah, it is considered Kefirah.
"They make the Divrei Torah a Refuas Haguf and it isn’t but a Refuas Hanefesh"
I heard from Harav Hagaon R. Ahron Lopiansky shlita,
of a farmer an Am Ha'aretz whose wife woke him up early in the morning to say Selichos. The husband asked her "what is this Selichos?"
His wife answered him "this is the special Tefillah we daven so that the cows should give us lots of milk and the chickens should lay every day extra large eggs."
Many of us think the same way and are not much different
than the farmer's wife. Some think, the only way to be Mekayam Mitzvahs Hafrashas Challah, one must do it together with 40 other women and have in mind a childless couple.
*(Hl. Avoda Zarah (Perek 11)