if your tank doesn't have a lot of flow in it, you do need to turn your balls once in a while and give them a gentle squeeze. yep, that's right, squeeze your balls people! lol. i personally give mine a hearty squish during a water change to extract all the built up mulm in and on them. they will float for a bit after a good squeeze but will sink back down after a day or so. turning them occasionally means they don't get brown on the underside where they don't get light.
i have one in a 2.5 gallon with a 13 watt spiral cfl, and three in a 55 with only 40 watts of t8 lighting. the ball in the 2.5 gal is kinda smushed into a corner of the tank so it doesn't move around with flow at all, but it is directly under the filter output so it gets 'washed' constantly, and shrimp pick at it all the time too. the three balls in the 55 tend to move around a bit in the filter flow, and just have ramshorn snails climbing on them now and then. the ball in the 2.5 is brighter green and the three in the 55 are darker, kinda like spinach green, so lighting definitely plays a part although they are considered low light.