Summing multiple different cells across different work sheets

Ok so am I going crazy????

I swear I used to be able to do this:

I was able to either use the sum function button or type the sum formula, I could then select multiple cells on sheet 1 I could then click sheet two and while holding ctrl I could select multiple cells on sheet 2 and push enter. these would auto sum for me.

as of right now i can't use the above process its like selecting the same cell across all sheets ???

the way i used to do it doesn't work anymore? am i going crazy or what - can someone help? when did this change?? or how do i fix this?

Hi
Use the sum function button or type the sum formula and select your first range, insert the separator char (comma or semicolon), change to new sheet and do the same.