If a balloon is filled with helium, why wouldn’t it float?

A balloon floats if the total weight of the balloon plus contents is less than the weight of air that it displaces.

If you were to put a completely massless stuff into the balloon it still must reach a size such that it displaces air equal in weight to the balloon.

When you put gas into the balloon it adds some mass so you need an even larger size before it will float.

If the gas is not pure hydrogen or helium the amount of weight added is such that the balloon could not float no matter how large it is.

Otherwise it is a matter that the inflated balloon is not large enough to reach that critical displacement.

You had it in a room field with hydrogen gas.

LOL. Sorry. I have no answer but you made me laugh. I pictured a confused 18-yr-old with those pointy birthday hats on and those flapper things you blow, trying to celebrate with your balloon and failing totally. xD

The uninflated balloon may simply be far too heavy; why not try using a simple party balloon first. Or, failing that, you can always try using a thin plastic bin bag and tie a knot at the end to seal it.

Otherwise, it's just possible that the gas is not helium as you think or is an impure form of helium.