I've been chumming with maggots for about 15 years and have never had this happen.
Remind me again how you got them. Did you buy them or grow your own? If you grew them, what kind of flies are they? Bluebottles (the kind I use), greenbottles and house flies all sink. If you grew them yourself, maybe you have something else.

Can you post a pic of some of them?
Here's something else to consider: what bedding are you storing them in? I use yellow corn meal. Among other things, the corn meal helps to "degrease" the maggots. Anything that small with grease on it will tend to float.
P.S. I saw your post on "Maggotdrowning". groundhog gives some advice similar to mine.