OK. I see the problem now. I'll go over a few issues and I'm sure others will chime in...
Wearing enough weight to "hold yourself down" doesn't necessarily mean that you were properly weighted. You could be under-weighted, over-weighted, or properly weighted. Also, being appropriately weighted and establishing proper horizontal trim are two issues that should be addressed separately.
First of all, you should know that as you use up the air in your tank, you will become more positively buoyant. With a typical 80 cuft tank, there's a 6 lb. weight swing from full to empty. With tanks that have a greater capacity, the weight swing will be even greater. You'll want to account for this weight swing when you perform a weight check. You can do this by: (1) doing the weight check with a near empty tank or (2) adding enough weight to compensate for any remaining air in your tank.
So how do you do a weight check? There are several different ways. The method taught by one of the major certifying agencies is to get in the water with all of your gear on (preferably a near empty tank) and no air in your BCD. While in a vertical position, add enough weight until the surface of the water is at eye level while you hold a normal breath. If you're tank wasn't empty during the weight check, then add enough weight to compensate for the remaining air. (For instance, if you have 1500psi remaining in a AL80 then add 3 lbs.) I recommend doing the weight check with a buddy so that he can hold onto the extra weight for you.
Why should you be properly weighted? Well, being properly weighted will make diving more comfortable and, more importantly, safer. Over-weighting is bad because you'll be dragging around extra weight with you on the dive. You'll have to add extra air into your BC to compensate for that extra weight. Managing a bigger bubble in your BC can make it more difficult to control your position in the water column. You'll also be less streamlined and more inefficient moving through the water.
Under-weighting is bad because it may prevent you from controlling your ascent properly and completing a safety stop at 15 fsw. During ascent, the three factors of your wetsuit becoming more positively buoyant, the air in your BC expanding, and consumption of tank air all contribute to you becoming more positively buoyant.
When properly weighted, you should be able to hover effortlessly at any depth you want...even with a near empty tank just a couple feet below the surface.
Once you get your weighting requirements sorted out, then you should distribute the weight appropriately to ensure good horizontal trim -- "skydiver" pose -- while in the water.
Whenever your gear configuration changes (new wetsuit, different tank, new drysuit undergarment, fresh to salt water, etc.), you should do another weight check. Record how much weight you needed and where you distributed that weight in your dive logbook.
I hope this info helps.
Have fun and dive safe...
