Copy and pasting formulas no longer working?

Ever have one of those moments where you feel like you're going insane? This is one of them.
I've been using Excel for about 15 years now, and I've come across a problem that apparently is very well known, but has *never* applied to me. Until now. And now it seems crazy.

I've just come to do some work on a new Excel sheet at home, and about 90% of this work is a copy and paste of several other sheets into another, new, workbook.

Now, I always, *always* open each work book in a separate Excel instance* (for 2 reasons**). And in the past I've always been able to copy a formula, or group of, and just paste them into a new workbook. Not today. Oh no, excel says no, can't be done. Googled the problem, and found out the same thing; apparently you cannot copy and paste forumlas from one instance of Excel into another; it'll just paste the values instead. Now, my PC here at home is doing exactly that (I swear it never did), however I just contacted a friend who I happened to know is using a PC that does this fine... and it does! It'll happily copy and paste formula from one instance to another.

So, basically, double-you tee eff!?

Any ideas?? (Running Excel 2010 on Win10 BTW)

*Just to clarify what I mean here. I'll open Excel, and open a file. I then right-click the excel icon on the Windows taskbar, and launch Excel again, and then open up book 2.

**As for always running separate instances of Excel, 1 for each workbook, here's why:
Reason 1; Alt-Tab. If you have 4 work books open in 1 instances then you can't alt tab between them (And other programs). This basically means using the mouse, and I'm loathe to use the mouse at all when using Excel cos it slows me down.
Reason 2; Screen Estate. If I need to use 2 books then I can alt tab between 2 of them very fast, and them be full screen. If they're tiled windowed in one instance, I've only got half a screen. Using dual monitors doesn't help either cos the toolbars are all streeeeetched out and start on the left hand side, and my monitors is on the left, not the right, meaning all of my tool bars are in the wrong place.

"It'll happily copy and paste formula from one instance to another."

Are you talking about copying the same workbooks on his PC or is he copying and pasting different workbooks?