I have an old Zeagle Scout BC with a 38-lb wing attached. I tried this out in a pool and had problems sinking because it wasn't deflating fully...
Assuming you're in a vertical orientation when deflating the BC and have trouble sinking -- and you verified that the BC is trapping noticeable air (you're not in need of a little more lead) -- then what ams511 is suggesting may be a quick fix to try. It seems unlikely to me that you are already immersed (at least 2-3 feet down), have assumed horizontal trim but can't vent the BC. If that's the case the suggestions by fisheater and herman are probably correct.
Assuming the former, the following two pics show the general idea of what you can do with bungee cords. It's for a sidemount BC with a donut shaped wing (Hollis SMS100) whose topside has been constricted with 3/8 in shock cords so that buoyancy is directed toward the hip area (a different concern from yours).
Because of the relative position of centers of gravity v. buoyancy, esp. with steel tanks, this is needed to achieve horitonal trim w/o overweighting.
In your case, you may want to apply even constriction everywhere and, if so, just tightening the bungee a bit may achieve the goal. If not, depending on available attachment points, use 3/8 bungees to apply uniform constriction and see if it helps. Don't apply too much pressure since you will be reducing some of the 38 lb lift capacity that you may need.
The bungee constriction method may or may not be a sufficient solution. If I open the zipper of the cordura BC enclosure and examine the plastic wing inside, it's crumpled up (before the constriction modification). At depth, as fisheater noted, ambient pressure will be your helper and press the air out. But in shallow depths, including when trying to sink, you don't have the full benefit of that and air trapped in the crumpled plastic wings may not easily vent. You will see some cave divers actually reach back and push down on their BC when this happens (to move air around). So one other option is to use your hands/arms to push inward on the BC when trying to sink (a third arm would be nice to hold the ventilator open but use one arm to apply some squeeze) and see if that helps.
Lastly, if the trapped air that prevents you from sinking is not much, then consider trying a couple of pounds of lead (but you don't want to overweight) to see if that does the trick. You don't want to be positively buoyant near the end of your dive. Hope this helps.
