Why can't I add/subtract pixels from selection?

Hi

I've recently started using PS CC 2017. I use it both at home and at work.

I often want to select pixels in one or more layers. At work I do the following:

• Command click the thumbnail in the layers panel which selects the pixels

• I then Add pixels from other layers to the selection by clicking command /shift on the layer thumbnail which brings up a little plus sign

• Similarly if I click command/option I get a little minus sign and I can subtract pixels from my selection

At work this is all fine. I use it regularly.

However, at home I'm unable to do this and it's driving me mad! I can select the pixels of one layer but when I try to Add pixels I don't get the little plus sign when I click command/shift. It flashes up for a split-second then disappears. Same problem applies for subtracting pixels. I don't get the minus sign, except for a glitchy flash then it disappears.

I spoke to Adobe support but they weren't helpful, telling me this wasn't possible in the new version of PS. However, I know it is because it functions fine at work - I tested literally minutes ago. But it doesn't work at home.

If my description wasn't clear it's explained here by Adobe: Load selections from a layer in Photoshop

Please help!!!!!

Thank you,

Claire