I have filled my balloon with helium, but it will not float. Why?
The reason may be that you have not filled the balloon with sufficient helium for its size; simply fill the balloon with more helium until it reaches the correct size. Your inflated balloon should yield slightly when you press on it.
If it is not flexible it means that your balloon is overfilled and may burst.
If you have added tassels and other decorations to your balloon string, it may be too heavy for the balloon to be able to carry. Try removing some of the decorations and this should help.
Finally, if your party is outside and the weather is very warm or cold, this can also affect the balloon and its optimal floating time.