What will prevent a drowned body from resurfacing?
A lot of times you hear of drowned bodies resurfacing and then there are many times when the body doesn't resurface at all. I am wondering what prevents a body from resurfacing.
The best way to approach this is to consider what makes a body surface, and then work backwards from there. Most people are nearly at neutral buoyancy when alive; if you float face down and slowly exhale, you will probably find that you begin to sink. If a person drowns, water may or may not fill the lungs, depending on whether the initial inhalation of water caused the epiglottis to close. If this happens, the person succumbs to what is called "dry drowning", which is essentially suffocation; in this case the lungs may have only a small amount of water in them, and the body will tend to float right away.
After the victim has died, his or her intestinal bacteria will remain active for some time, producing methane and other gasses which will be trapped in the digestive tract and will cause the body to bloat and become increasingly buoyant. How fast this happens is primarily determined by temperature, as the bacteria are adapted to warm interior of the body and will become less active as the body cools.
So to answer your question, adding weight to the body would of course keep it from floating up. A body submerged in very cold water would also be likely to stay down, and in temperate areas a fall or winter drowning victim may stay submerged for some time, only to float up when the water warms up in the summer months. If the gasses cannot become trapped inside the body it is also more likely to stay submerged, so a body which was injured in such a way as to open the intestinal cavity, either pre- or post-mortem, will be more likely to stay submerged.