The good fellas over at my local rental place know these old houses in the neighborhood inside and out and assured me that
this is normal and I just have to keep making passes but man that old finish is just so hard to get rid of in some points!
One way to get it out is to make a slight diagonal pass but then it's a bitch to get those marks out and back inline with the
grain.

Maybe I'm answering my own question here but should I wait until the finer grits to make the diagonals?
I'm just afraid once the shellac is gone I'll not know where the low points are, I'll miss them later and
the stain will not take nice in those spots. But one redeeming thing is that I plan on renting the random
pad sander when I return these pieces.

I wouldn't say it's cupping because it's not isolated to any individual board but over low points.
It's really not that thick a finish to begin with. I've read a lot on here about everyone starting out with
36 grit but they gave me sheets of 16 for a reason I guess.

Is this normal to have to keep making my passes until it comes out? If all the boards were level it'd be a cinch to run this
machine!

Also, as I'm crossing over from oak to maple, is it normal for maple to have these dark marks in some places?
They almost look like faint sharpie marks but they don't come out, I have to assume it's a natural effect of the wood.

Thanks in advance, and I must say I have an absolutely new profound respect for professional floor finishers. My back is feelin' it!!

Update:::

Well I don't know if the machine was designed this way but I'm gradually (graduated-like)
pulling the Clarke handle up toward my body as I pull it back, this takes a lot of weight off the wheels
and uses the weight of the machine to sand a lot harder down.

The guys at Jack's say there's plenty of thickness in these old 20's boards so unless there's a chance of wearing the
wood down too thin I think I'll be ok until i get the low points out...

All floor sanders are designed to sand floors flat. If a floor has low spots, it's common sense to understand those will be the last to get sanded, as the sanding machine is having to remove the high places around that low spot before it will even hit that low spot. That's one of the reasons why us pros use sanders with drum levers. We can exert more pressure on the lever when going over a certain area to apply more sanding pressure to the head, which will sand an area more aggressively. The EZ8 has this feature. So all you need to do is push down of the lever more when sanding those low spots. Too much and you'll dig a ditch, so be careful.

Thanks Gary you seem to be everywhere helping everybody!

Well, I've spent as much time as I can going over those low points and I've
decided the thing to do is to use the edger with the fine discs before the fine pass
with the drum. This will ensure that what I use to remove the shellac in there
will be the finest because once it's up I'll have idea where those low point are!

I'm assuming that when I get the orbital pad sander either with 150 or screens it'll be a little
more flexible to fit into those low points.

This is the one thing I didn't take into account! I shoulda been done yesterday.

UPDATE :::

I made a few more passes and then jumped to the medium paper (36)
and amazingly IT was able to get about 2/3 of those orange streaks out!

This is great, I'll finish it off as normal and ignore anything left over and finish it
off with a palm-sander!

On a side note, it's remarkable how easy it is to gum up the edger pads!