Updates!
OK, My father/pa/dad and I finally worked on this RCA ANT-751 antenna in the attic, one more time today, since its return date to Fry's Electronics is a few days away and today was the only good day to retry it.
We also worked on trying to get the multiple house rooms as well. We found the Dish's multiswitch and replaced it with an old splitter. However, we only got two rooms. We also finally got all the rooms's coax outlets working. We noticed the cables setups are messy and confusing in the attic. It seems like Dish or someone disconnected the old coax cables? We also found the best spot in the attic and hopefully, the channels will stay this way even though there are missing ones. We also brought in an old portable CRT TV, with its power AC, with a few years old Zeinth DTT900 converter box (got one with a government coupon), I ran channel scans and noted these working "stable" channels:
Old portable CRT TV + Zeinth DTT900 DTV converter box:
57-83 channels found (did two scans): KCBS2, KNBC4, KTLA5, KABC7, KCAL9, KSCI18, 22, KCET28, ION30, 31, 34, 46, 52, 56, 57, KLCS58 (soso), 62, and 63.
Portable DTV (ATSC; Digital Prism):
88 channels found (wow): KCBS2, KNBC4, KTLA5, KABC7, KCAL9, 22, 28, ION30, 31, 34, 46, 52, 56, 57 (soso), 58, and 62.
Note that I gave the channel the station code/ID names for the ones I care for.
As you can see both of them are missing the ones I care for: KTTV11 (I watch FOX's shows a lot and sports sometimes), KCOP13 (I used to watch it for its Star Trek, but don't care any more), and KOCE50 (rarely though).
I find it amusing with a Terk FDTV1a Digital Pro antenna (vertical flat one), that my father/da/pa bought from Best Buy which I did not approve [sighs], worked for these missing stations: KCAL9 and KTTV11. The catch is that this antenna has to be on the middle the kitchen floor (no furnitures yet!) facing the garage's wall with an old central air condition (AC) unit throuhg the big window doors. LOL! That makes no sense though! There's wall! Maybe reflective from the wall and/or AC? Of course, this antenna is useless for every channels for its weakness (duh, we knew that).
I wonder if there something else to try to get KTTV11 and KCOP13. Then, I really don't need cable/satellite service!

I do need to test the channels again to be sure they are really that stable. As you noticed, some scans don't get all or missed them. I even tried forcing them like 11-1, but no signals.