Your 200 undersuit will make alot of difference... but it shouldn't be that much.
I'd guess that the reason you couldn't sink could be partly to do with your breathing. Your lungs probably hold about 5l of air, breath it out and you'll become 5 kg heavier... don't breath in again for a while and you'll go straight down - if not add a bit more lead till you do.
Breathing wrong is the biggest mistake i come across - that and putting your fins on the wrong foot
