these things have loads of little like erm dips in the surface of them, this then creates little bubbles around them when you put them in the water, this then makes the wafer float.
if you quickly poke the wafer so it goes under the water (another way to explain it is hitting the wafer with your finger tip so it goes under the water really fast) this should make the air on the wafer come off and float to the top and burst. you wont see them burst coz they are really tiny. then your wafer should sink.
other wise, they should slightly begin to disolve and sink straight away.
i find that if you break them in half and hold them under the water for just about 10 seconds or something then they sink straight away.
but yea, for me they usually sink straight away.
