Depth
If you go down/up (descend/ascend) the gas spaces in your suit (dry or wet), your BCD and your body are shrinking/expanding, which decreases/incrases your buoyancy. To counter balance the buoyancy you have to fill/release gas into/from your BCD.
Time
During your dive you will consume the gas from your breathing tank/tanks, which will reduce its/their weight. To counter balance the buoyancy you have to release gas from your BCD.
To get over/under small obstacles prefer to use your lungs instead of the BCD for more precision and less gas consumption.