Problem in Excel

When I past figures into cells that are used in a formula, it will not autosum. ie;
if I have =A2+B2 in C2..when I past the A2 value, then it shows in C2 but when I past the value (from website) in B2 the C2 goes blank. I can CTrl D and it will fill in but one extra key stroke when I used this all day is frustrating.