It is amazing to me how many people do not respond- even when it is a formal invitation with a stamped response card.   In talking to other people and comparing notes it seem that about 30% of people do not respond.   The last two weeks before an occation you spend tracking people down and trying to get an answer from them.   

Sometimes it seems like they are waiting for a better answer.   Sometimes they can't make up their mind. Often they are just too lazy or don't want to tell you no.    When you have a formal occation and need to give a caterer a number which you are paying for it is real annoying.

Some people say when they don't respond you should assume it is no.   Others say when they don't respond you should assume it is yes.   I agree with the above comment. If they don't have the courtesy to respond, don't invite them again.