Pasting into Excel

In my job, I research the web and “copy” values (numbers) from lists which I paste into an Excel spreadsheet.
I have formulas in the sheet, ie; =A1+B1 (in C1).If I direct enter (keyboard) the values I find into the cells A1 and B1 then they calculate into C1 properly but if I “copy and paste” from the web page into the cells, C1 will not automaticly fill in. I can manually ‘Ctrl+D’ and it will fill in. As I paste into A1, that value shows up in C1 but when I paste into B1 then C1 goes blank. A small problem as one extra key stroke will cause it to fill in but as I do this all day long it would be nice if I can find the cure. Any thoughts